What is the efficacy of COVID-19 vaccinations in preventing disease transmission to the non-vaccinated?

The following information resources have been selected by the National Health Library and Knowledge Service Evidence Virtual Team in response to a question from the National Immunisation Advisory Committee (NIAC). The resources are listed in our estimated order of relevance to practicing healthcare professionals confronted with this scenario in an Irish context.  In respect of the evolving global situation and rapidly changing evidence base, it is advised to use hyperlinked sources in this document to ensure that the information you are disseminating to the public or applying in clinical practice is the most current, valid and accurate. For further information on the methodology used in the compilation of this document ⎯ including a complete list of sources consulted ⎯ please see our National Health Library and Knowledge Service Summary of Evidence Protocol.

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Main Points

  1. Emerging evidence suggests that COVID-19 vaccines may also reduce asymptomatic infection, and potentially transmission.
  2. In the Moderna trial, among persons who had received a first dose, the number of asymptomatic persons who tested positive for SARS-CoV-2 at their second-dose appointment was two-thirds lower among vaccine than among placebo recipients (0.1% and 0.3%, respectively). 
  3. Efficacy of the Janssen vaccine against asymptomatic seroconversion was 74% in a subset of trial participants.
  4. Preliminary data from Israel suggest that persons vaccinated with the Pfizer-BioNTech vaccine who develop COVID-19 have a 4-fold lower viral load than unvaccinated persons, which may indicate reduced transmissibility, as viral load has been identified as a prime driver of transmission; and that vaccination with Pfizer-BioNTech reduces viral load by 1.6 to 20 in individuals who are positive for SARS-CoV-2. 
  5. Preliminary data from Britain suggest that vaccination of healthcare workers with Pfizer-BioNTech or AstraZeneca was associated with a substantial reduction in COVID-19 cases in household contacts consistent with an effect of vaccination on transmission.
  6. Following vaccination with the Pfizer-BioNTech or AstraZeneca vaccine, the odds of a household contact of a vaccinated person developing SARS-CoV-2 infection were significantly lower when compared with the odds of a household contact of an unvaccinated person. 

Summary of Evidence

Evidence demonstrates that the authorized COVID-19 vaccines are both efficacious and effective against symptomatic, laboratory-confirmed COVID-19, including severe forms of the disease. In addition, a growing body of evidence suggests that COVID-19 vaccines may also reduce asymptomatic infection, and potentially transmission3

Rhesus macaque challenge studies provided the first evidence of the potential protective effects of Pfizer-BioNTech, Moderna and Janssen COVID-19 vaccines against SARS-CoV-2 infection, including asymptomatic infection. Vaccinated macaques developed neutralizing antibodies that exceeded those in human convalescent sera and showed no or minimal signs of clinical disease after SARS-CoV-2 challenge. In addition, COVID-19 vaccination prevented or limited viral replication in the upper and lower respiratory tracts, which may have implications for transmission of the virus among humans3

In human clinical trials, preliminary data suggest that COVID-19 vaccination may also protect against asymptomatic infection. In the Moderna trial, among persons who had received a first dose, the number of asymptomatic persons who tested positive for SARS-CoV-2 at their second-dose appointment was approximately two-thirds lower among vaccine than among placebo recipients (0.1% and 0.3%, respectively). Efficacy of the Janssen COVID-19 vaccine against asymptomatic seroconversion was 74% in a subset of trial participants3.

Preliminary analyses from the United States, Britain and Israel demonstrate that a two-dose mRNA COVID-19 vaccination series is highly effective against SARS-CoV-2 infection (including both symptomatic and asymptomatic infections). In the United States, the effectiveness of mRNA COVID-19 vaccination (either Pfizer-BioNTech or Moderna) was 89% against SARS-CoV-2 infection; vaccinated persons who were diagnosed with COVID-19 had a 60% lower hospitalization rate than unvaccinated persons. Among British healthcare personnel, Pfizer-BioNTech COVID-19 vaccination was 86% effective against SARS-CoV-2 infection. In British adults aged ≥ 80 years, including those with multiple underlying medical conditions, vaccine efficacy against symptomatic disease was estimated at 85%. In Israel, two doses of Pfizer-BioNTech COVID-19 vaccine was 90%–94% effective against a spectrum of illness: 

Irish and/or International Guidance

    Level 1

Royal College of Physicians of Ireland (RCPI). National Immunisation Advisory Committee (2021) Immunisation Guidelines: Chapter 5a: COVID-191

Pfizer/BioNTech: VACCINE EFFICACY

Data presented to the European Medicines Agency (EMA) demonstrated a two-dose vaccine efficacy of 95% (95% confidence interval of 90.3% to 97.6%) in those aged 16 and above. High efficacy was observed across, age, sex and ethnicity categories and among persons with underlying medical conditions.

Moderna: VACCINE EFFICACY

Data presented to the EMA demonstrated a two-dose vaccine efficacy of 94.1% (95% confidence interval of 89.3% to 96.8%) in those aged 18 and above. High efficacy was observed across age, sex and ethnicity categories and among persons with underlying medical conditions.

AstraZeneca: VACCINE EFFICACY

Data presented to the EMA demonstrated a two-dose vaccine efficacy of 59.5% (95% confidence interval of 45.8% to 69.7%) in those aged 18 and above. There was insufficient clinical data to allow reliable calculation of efficacy in those aged 55 and older. However, as a similar immune response was shown in all age groups, including those aged 65 and older, the EMA authorised the vaccine for all adults. The World Health Organization (WHO) Strategic Advisory Group of Experts (SAGE), subsequently reported the overall vaccine efficacy at 63.1%. There were no cases of COVID-19 hospitalisation, severe disease or death in those aged 65 and older who received the vaccine. 

    Level 1

Royal College of Physicians of Ireland (RCPI). National Immunisation Advisory Committee (2021) Clinical Guidance for COVID-19 Vaccination2

Pfizer/BioNTec: VACCINE EFFICACY

Data from the randomised Phase 3 trial demonstrated a two‐dose vaccine efficacy of 95% (95% confidence interval of 90.3% to 97.6%) in those aged 16 and above. Efficacy was similar in all age groups. 

Moderna: VACCINE EFFICACY

Data from the randomised Phase 3 trial demonstrated a two‐dose vaccine efficacy of 94.1% (95% confidence interval of 89.3% to 96.8%) in those aged 18 and above. Efficacy was similar in all age groups.  High efficacy (≥86%) was observed across age, sex and ethnicity categories and among persons with underlying medical conditions.

AstraZeneca: VACCINE EFFICACY

The EMA licensed documentation states that pooled analysis of the randomised Phase 2/3 trials demonstrated a two-dose vaccine efficacy of 59.5% (95% confidence interval of 45.8% to  69.7%) in those aged 18 and above. 

There was insufficient clinical data to allow reliable calculation of efficacy in those aged 55 and older. However, as a similar immune response was shown in all age groups, it is expected that a reduction in COVID-19 disease will be achieved in this age group. The EMA stated that the vaccine can be used in older adults. 

Evidence shows that protection starts from approximately 3 weeks after first dose of vaccine and persists up to 12 weeks. Studies show 76% protection overall against symptomatic COVID-19 disease in the first 90 days. Modelling showed no evidence of waning of protection in the first 3 months after vaccination. 

Higher efficacy of 82% after the second dose was found if the booster dose was given at 12 weeks. 

    Level 1

Centers for Disease Control and Prevention (United States) (2021) Background Rationale and Evidence for Public Health Recommendations for Fully Vaccinated People3

See Section: COVID-19 vaccine efficacy and effectiveness

The CDC points out that vaccine efficacy refers to how well a vaccine performs in a carefully controlled clinical trial, whereas effectiveness describes its performance in the real world. Evidence demonstrates that the authorized COVID-19 vaccines are both efficacious and effective against symptomatic, laboratory-confirmed COVID-19, including severe forms of the disease. In addition, a growing body of evidence suggests that COVID-19 vaccines may also reduce asymptomatic infection, and potentially transmission. Substantial reductions in SARS-CoV-2 infections (both symptomatic and asymptomatic) will have the positive benefit of helping to reduce overall levels of disease, and therefore, transmission. However, further investigations are ongoing to assess the impact of COVID-19 vaccination on transmission.

Animal challenge studies

disease, with evidence for protection against asymptomatic SARS-CoV-2 infection as well.

All authorized COVID-19 vaccines demonstrated efficacy (range 65% to 95%) against symptomatic laboratory-confirmed COVID-19. For each authorized COVID-19 vaccine, the overall efficacy was similar to the efficacy across different populations, including elderly and younger adults, in persons with and without underlying health conditions, and in persons representing different races and ethnicities.

All authorized COVID-19 vaccines demonstrated high efficacy (≥89%) against COVID-19 severe enough to require hospitalization.

All authorized COVID-19 vaccines demonstrated high efficacy against COVID-19 associated death. In the vaccine trials, no participants who received a COVID-19 vaccine died from COVID-19; the Moderna and Janssen trials each had COVID-19 deaths in the placebo arm.

Preliminary data suggest COVID-19 vaccination may also protect against asymptomatic infection. In the Moderna trial, among persons who had received a first dose, the number of asymptomatic persons who tested positive for SARS-CoV-2 at their second-dose appointment was approximately two-thirds lower among vaccines than among placebo recipients (0.1% and 0.3%, respectively). Efficacy of Janssen COVID-19 vaccine against asymptomatic seroconversion was 74% in a subset of trial participants.

No trials have compared efficacy between any of the authorized vaccines in the same study at the same time. All Phase 3 trials differed by calendar time and geography. Vaccines were tested in settings with different background COVID-19 incidence and circulating variants.

Real-world vaccine effectiveness

Preliminary analyses from the United States, Britain and Israel demonstrate that a two-dose mRNA COVID-19 vaccination series is highly effective against SARS-CoV-2 infection (including both symptomatic and asymptomatic infections). In the United States, the effectiveness of mRNA COVID-19 vaccination (either Pfizer-BioNTech or Moderna) was 89% against SARS-CoV-2 infection; vaccinated persons who were diagnosed with COVID-19 had a 60% lower hospitalization rate than unvaccinated persons. Among British healthcare personnel, Pfizer-BioNTech COVID-19 vaccination was 86% effective against SARS-CoV-2 infection. In British adults aged ≥ 80 years, including those with multiple underlying medical conditions, vaccine efficacy against symptomatic disease was estimated at 85%. In Israel, two doses of Pfizer-BioNTech COVID-19 vaccine was 90–94% effective against a spectrum of illness: asymptomatic SARS-CoV-2 infection, symptomatic COVID-19, as well as specifically severe COVID-19. Preliminary data from Israel suggest that persons vaccinated with Pfizer-BioNTech COVID-19 vaccine who develop COVID-19 have a four-fold lower viral load than unvaccinated persons. This observation may indicate reduced transmissibility, as viral load has been identified as a prime driver of transmission.

Vaccine performance against emerging SARS-CoV-2 variant strains

SARS-CoV-2 variants of concern (B.1.1.7 [first described in Britain]; B.1.351 [first described in South Africa]; P.1 [first described in Brazil]) have emerged with mutations that alter the receptor binding domain of the spike protein (notably the N501Y mutation occurring in all three variants, as well as E484K and E417T/N mutations in B.1.351 and P.1). These mutations appear to confer greater resistance to neutralization by sera from persons vaccinated with COVID-19 vaccines, raising concerns that these vaccines may have reduced effectiveness against COVID-19 illness, particularly against the B.1.351 variant. Therefore, vaccine performance against emerging SARS-CoV-2 variants is an important consideration when evaluating the need for continued prevention measures in vaccinated persons, and will require continued monitoring.

Immunogenicity

Sera from mRNA COVID-19 vaccine (both Pfizer-BioNTech and Moderna) recipients have generally demonstrated modest reductions in antibody neutralization activity against a variety of mutations; one study demonstrated poor neutralization activity for B.1.351. Across studies, the greatest reductions were observed for B.1.351, followed by P.1 and P.2 (another variant first described in Brazil); reductions for B.1.1.7 were minimal. The E484K mutation alone or in combination with other mutations in the receptor binding domain has been shown to account for the majority of reduction in vaccine-induced neutralizing antibody activity for the B.1351, P.1, and P.2 variants. For the Janssen viral vector COVID-19 vaccine, spike protein-specific antibody levels and seroresponse rates were similar between US clinical trial participants and participants from Brazil and South Africa, where the viral variants were circulating. In the absence of a biological correlate of protection, it is difficult to predict how reduced immunogenicity may affect COVID-19 vaccine effectiveness. However, across studies, antibody neutralizing activity of sera from vaccinated persons was still generally higher than that observed for convalescent sera from persons who have recovered from COVID-19.

Efficacy and effectiveness

As described above, preliminary results from Britain demonstrate that vaccination with two doses of Pfizer-BioNTech COVID-19 vaccine was highly effective (85–86%) against SARS-CoV-2 infection and symptomatic COVID-19 during a period when B.1.1.7 was the predominant circulating strain. Similarly, high Pfizer-BioNTech vaccine effectiveness (92%) against infection was observed in Israel in the context of multiple circulating strains, with the proportion of cases due to the B.1.1.7 variant increasing to 80% towards the end of the evaluation period. Preliminary data suggest that the Janssen COVID-19 vaccine may have reduced overall efficacy against the B.1.351 variant. In the United States, efficacy was 74% and in Brazil (where ~69% of infections were due to P.2) efficacy was 66%, but in South Africa (~where 95% of infections were due to B.1.351) efficacy was 52%. However, Janssen vaccine efficacy against severe or critical disease was high and similar across sites (73–82%).

    Level 1

Joint Committee on Vaccination and Immunisation (Great Britain) (2021) Interim Statement on Phase 2 of the COVID-19 Vaccination Programme4

There is emerging evidence that vaccination may prevent asymptomatic infection, which may be inferred as evidence of an impact on transmission. However, while these data are very encouraging, they are still limited and currently there is no strong real-world evidence of an impact of vaccination on transmission.

Furthermore, which groups contribute to viral transmission is currently not well defined. Data on mixing patterns generally indicate that people mainly mix with others of the same age group, with a higher frequency of contact than with those who are younger. In addition, members of certain occupations are required to interact with the general public to a lesser or greater degree, or have frequent close contact with co-workers. However, at present there is limited evidence to indicate that any particular age groups or occupations are associated with higher levels of transmission of SARS-CoV2 infection.

Mathematical modelling of strategies for phase 2 vaccination indicates that a transmission-based approach affords modest benefit at best. Speed of vaccine deployment is the single most important factor for an optimal programme that maximises public health benefit. As evidenced in phase 1, a simple age-based programme is considered the keystone of rapid vaccine deployment. Maintaining this structure through phase 2 of the programme will enable the continued high pace of vaccine deployment.

Evidence Synopsis Resources

    Level 2

BMJ Best Practice (2021) Coronavirus disease 2019 (COVID-19)5

BMJ Best Practice states that it is unknown whether vaccines prevent asymptomatic infection or transmission from individuals who are infected despite vaccination. Vaccinated people should continue to follow public health recommendations. Safety and efficacy ⎯ including duration of immunity ⎯ beyond 2 months is unknown. Advice will be updated as information on the impact of vaccination on virus transmission and indirect protection in the community is assessed.

BMJ Best Practice note that in the US, the Centers for Disease Control and Prevention recommend that people who have been vaccinated do not need to quarantine after exposure to a person with COVID-19 provided that they meet the following criteria: they have been fully vaccinated and at least 2 weeks have passed since the second dose in a 2-dose series or at least 2 weeks have passed since the dose of a single-dose vaccine; they are within 3 months of their last dose in the series; and they have remained asymptomatic since their current exposure.

BMJ Best Practice states that emerging unpublished evidence suggests real-world efficacy in reducing the rate of infection, disease severity, hospitalisation and asymptomatic infection; however, these studies have not been peer reviewed or published as yet and further research is required.

BMJ Best Practice provides a table comparing the vaccines that have been authorised to date in Britain, Europe and the US. In addition to these, CoronaVac® and Sinopharm® (inactivated version of the severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] virus) have been authorised in China, and Sputnik V® (an adenovirus vector vaccine) has been authorised in Russia and is 91.6% effective.[355] Several other vaccine candidates are still in development including mRNA vaccines, DNA vaccines, viral vector vaccines, protein subunit vaccines, live-attenuated vaccines, inactivated virus vaccines, and intranasal delivery systems.

 Click to view in source document.

The efficacy of the authorised vaccines is given as: 

  • Pfizer/BioNTech: 95%
  • Moderna: 94%
  • AstraZeneca: 55% to 80% depending on dose interval
  • Janssen: 66%


    Level 2

UpToDate (2021) Coronavirus Disease 2019 (COVID-19) Vaccines to Prevent SARS-CoV-26

See Section: IMMUNOGENICITY, EFFICACY AND SAFETY OF SELECT VACCINES

UpToDate notes that the vaccines included in this section have elicited neutralizing and cellular responses in nonhuman primates without evidence of enhanced disease. They have demonstrated immunogenicity in early-phase human trials, most of which compared receptor-binding antibody and neutralizing antibody titers to those found in serum from patients convalescing from prior SARS-CoV-2 infection (ranging from asymptomatic to severe infection). It is difficult to compare the immunogenicity of the different vaccine candidates based on these studies, in part because of the heterogeneity of the assays employed. None of the early trials identified major safety concerns, but all vaccines elicited systemic adverse effects ⎯ fever, chills, headache, fatigue, myalgia, joint pains ⎯ in a proportion of participants, some of whom rated the effects severe enough to limit daily activity.

Although results of phase III efficacy trials have been reported for several COVID-19 vaccine candidates and indicate efficacy in preventing symptomatic COVID-19, many uncertainties remain. These include vaccine effects on microbiologically confirmed SARS-CoV-2 infection (both asymptomatic and symptomatic, which could have implications for transmission) as well as the durability of protective effect. 

None of the vaccines have been studied against one another, and thus, comparative efficacy is uncertain. Differences in the magnitudes of effect reported from phase III trials might be related to factors other than effectiveness, including differences in the trial populations and locations, timing of the trials during the pandemic, and study design.

Pfizer BioNTech: EFFICACY

In a large placebo-controlled phase III trial, this vaccine had 95 percent efficacy (95% CI 90.3-97.6) in preventing symptomatic COVID-19 at or after day 7 following the second dose. This effect was assessed after an analysis of 170 confirmed COVID-19 cases ⎯ 8 in the vaccine group and 162 in the placebo group ⎯ among over 36,000 participants aged 16 years and older with a median of 2-month follow-up after vaccination. Nine of the 10 severe cases that occurred during the study were in the placebo group. Among adults ≥65 years who had other medical comorbidities or obesity, vaccine efficacy was 91.7 percent (95% CI 44.2-99.8). Among the entire trial population, the rate of COVID-19 in the vaccine group started to decrease relative to the rate in the placebo group approximately two weeks after the first dose (estimated vaccine efficacy of 52 percent, 95% CI 29.5-68.4 between the two doses).

Observational data from various countries following their national roll-outs of BNT162b2 support the trial findings. In a study from Israel that included nearly 570,000 BNT162b2 recipients and an equal number of matched unvaccinated controls, estimated vaccine effectiveness seven days or more following the second dose was 92 percent for documented SARS-CoV-2 infection, 94 percent for symptomatic COVID-19, 92 percent for severe COVID-19, and 87 percent for COVID-19 resulting in hospitalization. Corresponding vaccine effectiveness rates during the period of 14 to 20 days after the first dose were 56, 57, 62, and 74 percent. In another study of 9109 health care workers in Israel, among whom 79 percent received at least one vaccine dose, the adjusted risk reduction in SARS-CoV-2 infection was 30 percent in days 1 to 14 after the first dose and 75 percent in days 15 to 28. In an unpublished study of over 23,000 health care workers in Britain, covering a time frame when the B.1.1.7 variant was prevalent, vaccine effectiveness against SARS-CoV-2 infection (both asymptomatic and symptomatic) was estimated at 72 percent 21 days or more after the first dose and 86 percent 7 days or more after the second dose.

Although these data suggest some efficacy of a single vaccine dose, the actual magnitude and duration of protection from a single dose are unknown because most participants in the trial and the observational studies received the second dose three weeks after the first.

Moderna: EFFICACY

In a large placebo-controlled phase III results, mRNA-1273 had 94.1 percent vaccine efficacy (95% CI 89.3-96.8) in preventing symptomatic COVID-19 at or after 14 days following the second dose. This effect was assessed after an analysis of 196 confirmed COVID-19 cases ⎯ 11 in the vaccine group and 185 in the placebo group ⎯ among nearly 30,000 study participants aged 18 years and older with a median follow-up of two months after vaccination. Among adults ≥65 years of age, vaccine efficacy was 86.4 percent (95% CI 61.4-95.5). Thirty cases were severe, and all of these occurred in the placebo group. Among approximately 2000 participants who only received a single dose of vaccine or placebo, vaccine efficacy following the dose was 80.2 percent (95% CI 55.2-92.5); however, these individuals only had a median follow-up time of 28 days, so the duration of protection from a single dose remains uncertain. A preliminary analysis also suggested a reduction in asymptomatic infections between dose 1 and 2.

Janssen: EFFICACY

In a phase III efficacy trial detailed in briefing materials presented to the US Food and Drug Administration (FDA), Ad26.COV2.S, given as a single dose, had 66.9 percent efficacy (95% CI 59.0-73.4) in preventing moderate to severe/critical COVID-19 ⎯ which included patients with pneumonia, dyspnea, tachypnea, or at least two symptoms of COVID-19 ⎯ starting at or after 14 days following vaccination. This effect was assessed after an analysis of 464 confirmed moderate to severe/critical COVID-19 cases (116 in the vaccine group and 348 in the placebo group) among nearly 40,000 study participants aged 18 years and older with a median follow-up of two months after vaccination. There were only four mild COVID-19 cases. Vaccine efficacy starting at 28 days after vaccination was similar to that after 14 days. Vaccine efficacy against severe/critical disease trended higher at 78 and 85 percent after 14 and 28 days post-vaccination, respectively.

Reported overall efficacy varied by region: 74 percent in the United States, 66 percent in Brazil, where the P.2 variant was prevalent, and 52 percent in South Africa, where most infections were caused by the variant B.1.351. Nevertheless, vaccine efficacy against severe/critical disease was similar across regions, and in South Africa was 72 and 83 percent after 14 and 28 days post-vaccination.

AstraZeneca: EFFICACY

In a report of interim results from a multinational phase III randomized trial, this vaccine had 70.4 percent efficacy (95% CI 54.8-80.6) in preventing symptomatic COVID-19 at or after 14 days following the second dose. This effect was assessed after an analysis of 131 confirmed COVID-19 cases ⎯ 30 in the vaccine group and 101 in the control group ⎯ among over 11,000 participants with a median follow-up of two months after vaccination. Ten participants were hospitalized for COVID-19, including two who were categorized as having severe disease; all of them were in the control group. A subgroup of participants inadvertently received a lower vaccine dose for the first of the two vaccine doses, and the overall vaccine efficacy in this subgroup differed from the rest. Vaccine efficacy was 90.0 percent (95% CI 67.4-97.0) among the 2741 participants who received the lower dose and 62.1 percent (95% CI 41-75.7) among those who received full-dose vaccine. Reasons for this difference are uncertain, although the overlapping confidence intervals indicate that the difference is not statistically significant. Differences in the control administered (meningococcal vaccine for both doses at some study sites versus meningococcal vaccine for one dose with saline for another dose at other sites) and in the interval between administration of the two vaccine doses further contribute to uncertainty about the findings.

In a subsequent analysis of this trial, vaccine efficacy for symptomatic COVID-19 was 76 percent from 21 days after receipt of the first dose until receipt of the second dose or day 90, whichever came first, suggesting protection with a single dose. Additionally, receipt of the second dose at 12 weeks or later was associated with higher vaccine efficacy than receipt at <6 weeks (81 versus 55 percent). These findings lend support to extending the time interval for the second dose to 12 weeks.

According to another unpublished report, vaccine efficacy against B.1.1.7, a viral variant that has emerged as the dominant variant in Britain and has been identified in other countries, compared with other variants was not statistically different (75 versus 84 percent), despite induction of lower neutralizing activity against the B.1.1.7 variant. However, according to preliminary, unpublished results of a phase I/II trial in South Africa, the AstraZeneca vaccine did not reduce the rate of mild to moderate COVID-19 (at least one symptom but no tachypnea, hypoxia, or organ failure) over a time frame when B.1.351 was the dominant variant circulating. Because the trial was small and the number of cases was low, the estimate of vaccine efficacy had very wide confidence intervals (21.9 percent, 95% CI -49.9 to 59.8). Impact on severe disease, which was rare in the young, healthy trial population, could not be assessed.

Novavax: EFFICACY

In a press release concerning a phase III efficacy trial, NVX-CoV2373 had 89.3 percent (95% CI 75.2-95.4) efficacy in preventing symptomatic COVID-19 starting at or after seven days following the second dose in seronegative individuals. This effect was assessed after interim analysis of 62 confirmed COVID-19 cases ⎯ 6 in the vaccine group and 56 in the placebo group ⎯ among over 15,000 individuals aged 18 to 84 years. Only one of those cases was severe and occurred in the placebo group. The vaccine appeared to be highly effective against the variant B.1.1.7. However, in a smaller trial in South Africa, where most COVID-19 cases were caused by the B.1.351 variant, vaccine efficacy appeared lower, at 49.4 percent (95% CI 6.1-72.8). Serious adverse events occurred at similar rates in the vaccine and placebo groups.

Other Vaccines

Sputnik V: EFFICACY

This is a vaccine developed in Russia that uses two replication-incompetent adenovirus vectors that express a full-length spike glycoprotein. The vaccine is given intramuscularly as an initial adenovirus 26 vector dose followed by an adenovirus 5 vector boosting dose 21 days later. This vaccine is available in Russia and several other countries.

In an open-label, nonrandomized phase I/II trial, SARS-CoV-2 humoral and cellular immune responses were detected in the participants.

According to interim analysis of a phase III trial that included over 20,000 participants without serologic evidence of prior SARS-CoV-2 infection, this vaccine had 91.6 percent (95% CI 85.6-95.2) efficacy in preventing symptomatic COVID-19 starting at 21 days following the first dose [at the time of the second dose]. This effect was assessed after 78 cases of COVID-19: 16 of 14,964 participants who received vaccine and 62 of 4902 participants who received placebo. All 20 cases of severe COVID-19 that occurred 21 days after the first dose were in the placebo group. Median follow-up time was 48 days after the first dose. Local and systemic, flu-resembling reactions were more common in the vaccine group, at rates of 15 and 5 percent, respectively. No serious adverse events were deemed related to vaccine.

Sinopharm

This is an inactivated vaccine based on a SARS-CoV-2 isolate from a patient in China; it has an aluminum hydroxide adjuvant. The vaccine is given intramuscularly in two doses 28 days apart. In phase I/II placebo-controlled randomized trials of healthy individuals 18 to 80 years old, all recipients of two vaccine doses developed neutralizing and binding antibodies; no severe reactions were reported. This vaccine is available in China and some other countries.

Sinovac

This inactivated COVID-19 vaccine was developed in China; it has an aluminum hydroxide adjuvant. The vaccine is given intramuscularly in two doses 28 days apart. In phase I/II randomized, placebo-controlled trials, the vaccine appeared safe and immunogenic in healthy individuals aged 18 to 59 years as well as those 60 years or older. This vaccine is available in China and some other countries, including Brazil.

Table: Characteristics of select COVID-19 vaccines [Click to view in source document and/or print or export to PowerPoint]

Irish and/or International Literature

    Level 2

Voysey et al (2021) [Randomized Controlled Trial] Single-dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine: a pooled analysis of four randomised trials7

The authors provide a pooled analysis of trials of the ChAdOx1 nCoV-19 AstraZeneca vaccine and exploratory analyses of the impact on immunogenicity and efficacy of extending the interval between priming and booster doses. In addition, the authors show the immunogenicity and protection afforded by the first dose, before a booster dose has been offered.

METHODS: Data from three single-blind randomised controlled trials ⎯ one phase 1/2 study in Britain (COV001), one phase 2/3 study in Britain (COV002), and a phase 3 study in Brazil (COV003) ⎯ and one double-blind phase 1/2 study in South Africa (COV005) are presented. Individuals 18 years and older were randomly assigned 1:1 to receive two standard doses of ChAdOx1 nCoV-19 (5 × 1010 viral particles) or a control vaccine or saline placebo. In one trial, a subset of participants received a lower dose (2·2 × 1010 viral particles) of the ChAdOx1 nCoV-19 for the first dose. The primary outcome was virologically confirmed symptomatic COVID-19 disease, defined as a nucleic acid amplification test (NAAT)-positive swab combined with at least one qualifying symptom (fever ≥37·8°C, cough, shortness of breath, or anosmia or ageusia) more than 14 days after the second dose. Secondary efficacy analyses included cases occurring at least 22 days after the first dose. All cases of COVID-19 with a NAAT-positive swab were adjudicated for inclusion in the analysis by a masked independent endpoint review committee. The primary analysis included all participants who were SARS-CoV-2 N protein seronegative at baseline, had at least 14 days of follow-up after the second dose, and had no evidence of previous SARS-CoV-2 infection from NAAT swabs.

FINDINGS: Between April 23 and Dec 6, 2020, 24 422 participants were recruited and vaccinated across the four studies, of whom 17 178 were included in the primary analysis (8597 receiving ChAdOx1 nCoV-19 and 8581 receiving control vaccine). The data cut-off for these analyses was Dec 7, 2020. 332 NAAT-positive infections met the primary endpoint of symptomatic infection more than 14 days after the second dose. Overall vaccine efficacy more than 14 days after the second dose was 66·7% (95% CI 57·4-74·0), with 84 (1·0%) cases in the 8597 participants in the ChAdOx1 nCoV-19 group and 248 (2·9%) in the 8581 participants in the control group. There were no hospital admissions for COVID-19 in the ChAdOx1 nCoV-19 group after the initial 21-day exclusion period, and 15 in the control group. 108 (0·9%) of 12 282 participants in the ChAdOx1 nCoV-19 group and 127 (1·1%) of 11 962 participants in the control group had serious adverse events. There were seven deaths considered unrelated to vaccination (two in the ChAdOx1 nCov-19 group and five in the control group), including one COVID-19-related death in one participant in the control group. Exploratory analyses showed that vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 after vaccination was 76·0% (59·3-85·9). Our modelling analysis indicated that protection did not wane during this initial 3-month period. Similarly, antibody levels were maintained during this period with minimal waning by day 90 (geometric mean ratio [GMR] 0·66 [95% CI 0·59-0·74]). In the participants who received two standard doses, after the second dose, efficacy was higher in those with a longer prime-boost interval (vaccine efficacy 81·3% [95% CI 60·3-91·2] at ≥12 weeks) than in those with a short interval (vaccine efficacy 55·1% [33·0-69·9] at <6 weeks). These observations are supported by immunogenicity data that showed binding antibody responses more than two-fold higher after an interval of 12 or more weeks compared with an interval of less than 6 weeks in those who were aged 18-55 years (GMR 2·32 [2·01-2·68]).

INTERPRETATION: These results confirm that the vaccine is efficacious, with results varying by dose interval in exploratory analyses. A 3-month dose interval might have advantages over a programme with a short dose interval for roll-out of a pandemic vaccine to protect the largest number of individuals in the population as early as possible when supplies are scarce, while also improving protection after receiving a second dose.

    Level 2

Emary et al (2021) [Randomized Controlled Trial] Efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine against SARS-CoV-2 variant of concern 202012/01 (B.1.1.7): an exploratory analysis of a randomised controlled trial8

Volunteers (aged ≥18 years) who were enrolled in phase 2/3 vaccine efficacy studies in Britain and randomly assigned (1:1) to receive the AstraZeneca COVID-19 vaccine or a meningococcal conjugate control (MenACWY) vaccine provided upper airway swabs on a weekly basis. Swabs were tested by nucleic acid amplification test (NAAT) for SARS-CoV-2. The efficacy analysis included symptomatic COVID-19 in seronegative participants with a NAAT positive swab more than 14 days after a second dose of vaccine. Participants were analysed according to vaccine received. Vaccine efficacy was calculated as 1 – relative risk (ChAdOx1 nCoV-19 vs MenACWY groups) derived from a robust Poisson regression model. 

Viral load among those with a NAAT-positive swab in the ChAdOx1 nCoV-19 vaccinated group was statistically significantly lower than among those in the control group. Minimum Ct values from participants who received ChAdOx1 nCoV-19 (median 28·8; IQR 20·5–33·5) were higher than in those who received the control vaccine (20·2; 15·5–29·6; p<0·0001). Symptomatic participants who received ChAdOx1 nCoV-19 had a shorter NAAT-positive period (median 1·0 week; IQR 1·0–2·0) than those who received the control vaccine (2·0 weeks; 1·0–3·0). These findings suggest that even those vaccines with a NAAT-positive swab could be less likely to transmit the virus than an unvaccinated NAAT-positive individual. Vaccination reduced viral load and length of NAAT positivity against both B.1.1.7 and non-B.1.1.7 lineages. 

    Level 4

Hall and Foulkes (2021) [Preprint] Effectiveness of BNT162b2 mRNA Vaccine Against Infection and COVID-19 Vaccine Coverage in Healthcare Workers in England, Multicentre Prospective Cohort Study (the SIREN Study)9

Methods: The SIREN study is a prospective cohort study among staff working in publicly funded hospitals. Baseline risk factors, vaccination status (from 8/12/2020 to 5/2/2021), and symptoms are recorded at 2 weekly intervals and all SARS-CoV-2 polymerase chain reaction (PCR) and antibody test results documented. A mixed effect proportional hazards frailty model using a Poisson distribution was used to calculate hazard ratios to compare time to infection in unvaccinated and vaccinated participants in order to estimate the impact of the BNT162b2 vaccine on all [asymptomatic and symptomatic] infection.
Findings: A single dose of BNT162b2 vaccine demonstrated vaccine effectiveness of 72% (95% CI 58-86) 21 days after the first dose, and 86% (95% CI 76-97) 7 days after the second dose in the antibody negative cohort.
Conclusion: Our study demonstrates that the BNT162b2 vaccine effectively prevents both symptomatic and asymptomatic infection in working age adults. This cohort was vaccinated when the dominant variant in circulation was B1.1.7 and demonstrates effectiveness against this variant.

    Level 4

Tande et al (2021) Impact of the COVID-19 Vaccine on Asymptomatic Infection Among Patients Undergoing Pre-Procedural COVID-19 Molecular Screening10

Methods:The authors conducted a retrospective cohort study of consecutive, asymptomatic adult patients (n = 39,156) within a large United States healthcare system who underwent 48,333 pre-procedural SARS-CoV-2 molecular screening tests between December 17, 2020 and February 8, 2021. The primary exposure of interest was vaccination with at least one dose of an mRNA COVID-19 vaccine. The primary outcome was relative risk of a positive SARS-CoV-2 molecular test among those asymptomatic persons who had received at least one dose of vaccine, as compared to persons who had not received vaccine during the same time period. Relative risk was adjusted for age, sex, race/ethnicity, patient residence relative to the hospital (local vs. non-local), healthcare system regions, and repeated screenings among patients using mixed effects log-binomial regression.

Results:Positive molecular tests in asymptomatic individuals were reported in 42 (1.4%) of 3,006 tests performed on vaccinated patients and 1,436 (3.2%) of 45,327 tests performed on unvaccinated patients (RR=0.44 95% CI: 0.33-0.60; p<.0001). Compared to unvaccinated patients, the risk of asymptomatic SARS-CoV-2 infection was lower among those >10 days after 1st dose (RR=0.21; 95% CI: 0.12-0.37; p<.0001) and >0 days after 2nd dose (RR=0.20; 95% CI: 0.09-0.44; p<.0001) in the adjusted analysis.

Conclusions:COVID-19 vaccination with an mRNA-based vaccine showed a significant association with a reduced risk of asymptomatic SARS-CoV-2 infection as measured during pre-procedural molecular screening. The results demonstrate the impact of the vaccines on reduction in asymptomatic infections supplementing the randomized trial results on symptomatic patients.

    Level 4

Heymann and Zacay (2021) [Preprint] BNT162b2 Vaccine Effectiveness in Preventing Asymptomatic Infection with SARS-CoV-2 Virus: A Nationwide Historical Cohort Study11

Background: There is strong evidence regarding the efficacy and effectiveness of BNT162b2 vaccine in preventing symptomatic infection with SARS-CoV-2 virus. There is a paucity of data regarding effectiveness in prevention of asymptomatic infection.
Methods: In this real-world study, the authors identified a sub-population of patients in a large health maintenance organisation who were repeatedly tested for SARS-CoV-2 infection using a PCR test. These patients were used as the cohort for the study, and compared individuals who were vaccinated with BNT162b2 mRNA vaccine to unvaccinated individuals. A positive SARS-CoV-2 PCR test result was used as the outcome. The follow-up period was from January 1 until February 11, 2021.
Findings: 6,286 individuals were included in the cohort. Seven days following the second vaccine dose, a rate of 6 positive PCR tests per 10,000 patient-days was recorded, compared with a rate of 53 positive tests per 10,000 patient-days for the unvaccinated group. The estimated vaccine effectiveness against infection with SARS-CoV-2 virus after two vaccine doses was 89% (confidence interval 82%-94%). The estimated effectiveness two weeks following the first vaccine dose was 61% (confidence interval 49%-71%).
Interpretation: In this study, vaccination with BNT162b2 reduced infection rates among individuals who underwent screening by frequent SARS-CoV-2 PCR testing. Using a cohort of frequently tested individuals reduced the indication bias for PCR testing, which enabled estimation of asymptomatic infection rates.

    Level 4

Monge et al (2021) [Preprint] Direct and indirect effectiveness of mRNA vaccination against SARS-CoV-2 infection in long-term care facilities in Spain12

Objectives: To estimate indirect and total (direct plus indirect) effects of COVID-19 vaccination in residents in long-term care facilities (LTCF).

Design: Registries-based cohort study including all residents in LTCF ≥65 years offered vaccination between 27 December 2020 and 10 March 2021. Risk of SARS-CoV-2 infection following vaccination was compared with the risk in the same individuals in a period before vaccination. Risk in non-vaccinated was also compared to a period before the vaccination programme to estimate indirect protection. Standardized cumulative risk was computed adjusted by previous documented infection (before the start of follow-up) and daily-varying SARS-CoV-2 incidence and reproductive number.

Participants: 573,533 records of 299,209 individuals in the national vaccination registry were selected; 99.0% had ≥1 vaccine-dose, 99.8% was Pfizer/BioNTech (BNT162b2). Residents mean age was 85.9, 70.9% were females. A previous SARS-CoV-2 infection was found in around 25% and 13% of participants, respectively, at the time of vaccine offer and in the reference period.

Main outcome measures: Documented SARS-CoV-2 infection identified in the national COVID-19 laboratory registry.

Results: Total VE was 57.2% (95% Confidence Interval: 56.1%-58.3%), and was highest ≥28 days after the first vaccine-dose (proxy of ≥7 days after the second dose) and for individuals naïve to SARS-CoV-2 [81.8% (81.0%-82.7%)] compared to those with previous infection [56.8% (47.1%-67.7%)]. Vaccination prevented up to 9.6 (9.3-9.9) cases per 10.000 vaccinated per day; 11.6 (11.3-11.9) if naïve vs. 0.8 (0.5-1.0) if previous infection. Indirect protection in the non-vaccinated could only be estimated for naïve individuals, at 81.4% (73.3%-90.3%) and up to 12.8 (9.4-16.2) infections prevented per 10.000 indirectly protected per day.

Conclusions: Our results confirm the effectiveness of mRNA vaccination in an institutionalized elderly population; endorse the policy of universal vaccination in this setting, including in people with previous infection; and suggest that even non-vaccinated individuals benefit from indirect protection.

Key messages

  • COIVD-19 vaccination reduced the risk of documented SARS-CoV-2 infection in institutionalized elderly by 57.2% (56.1% to 58.3%), which increased to 81.2% (80.2% to 82%) for the fully vaccinated.
  • In individuals naïve to SARS-CoV-2 vaccination reduced the risk by up to 81.8% and averted up to 11.6 cases per 10,000 vaccinated persons per day.
  • Those with previous infection also benefited from a risk reduction of 57%, which translated in less than 1 infection averted per 10,000 vaccinated persons per day.
  • Non-vaccinated individuals living in facilities where the majority (residents and staff) had been vaccinated showed a risk reduction similar to those actually vaccinated.
    Level 4

Shrotri et al (2021) [Preprint] Vaccine effectiveness of the first dose of ChAdOx1 nCoV-19 and BNT162b2 against SARS-CoV-2 infection in residents of Long-Term Care Facilities13

Background: The effectiveness of SARS-CoV-2 vaccines in frail older adults living in Long-Term Care Facilities (LTCFs) is uncertain. We estimated protective effects of the first dose of ChAdOx1 and BNT162b2 vaccines against infection in this population.

Methods: Cohort study comparing vaccinated and unvaccinated LTCF residents in England, undergoing routine asymptomatic testing (8 December 2020 – 15 March 2021). We estimated the relative hazard of PCR-positive infection using Cox proportional hazards regression, adjusting for age, sex, prior infection, local SARS-CoV-2 incidence, LTCF bed capacity, and clustering by LTCF.

Results: Of 10,412 residents (median age 86 years) from 310 LTCFs, 9,160 were vaccinated with either ChAdOx1 (6,138; 67%) or BNT162b2 (3,022; 33%) vaccines. A total of 670,628 person days and 1,335 PCR-positive infections were included. Adjusted hazard ratios (aHRs) for PCR-positive infection relative to unvaccinated residents declined from 28 days following the first vaccine dose to 0·44 (0·24, 0·81) at 28-34 days and 0·38 (0·19, 0·77) at 35-48 days. Similar effect sizes were seen for ChAdOx1 (aHR 0·32 [0·15-0·66] and BNT162b2 (aHR 0·35 [0·17, 0·71]) vaccines at 35-48 days. Mean PCR cycle threshold values were higher, implying lower infectivity, for infections ≥28 days post-vaccination compared with those prior to vaccination (31·3 vs 26·6, p<0·001).

Interpretation: The first dose of BNT162b2 and ChAdOx1 vaccines was associated with substantially reduced SARS-CoV-2 infection risk in LTCF residents from 4 weeks to at least 7 weeks.

    Level 5

Lopez Bernal et al (2021) [Preprint] Early effectiveness of COVID-19 vaccination with BNT162b2 mRNA vaccine and ChAdOx1 adenovirus vector vaccine on symptomatic disease, hospitalisations and mortality in older adults in England14

OBJECTIVES: To estimate the real-world effectiveness of the BNT162b2 vaccine and ChAdOx1 vaccine against confirmed COVID-19, hospitalisations and deaths.

PARTICIPANTS: All adults in England aged 70 years and older (over 7.5 million). All COVID-19 tests in the community among eligible individuals who reported symptoms between 8 December 2020 and 19 February 2021 were included in the analysis.

INTERVENTIONS: One and two doses of BNT162b2 vaccine. One dose of ChAdOx1 vaccine.

MAIN OUTCOME MEASURES: Symptomatic PCR confirmed SARS-CoV-2 infection, hospitalisation and death with COVID-19.

RESULTS: Individuals aged ≥80 years vaccinated with BNT162b2 prior to 4 January had a higher odds of testing positive in the first 9 days after vaccination (odds ratio up to 1.48, 95%CI 1.23-1.77), indicating that those initially targeted had a higher underlying risk of infection. Vaccine effectiveness was therefore estimated relative to the baseline post-vaccination period. Vaccine effects were noted from 10-13 days after vaccination, reaching an effectiveness of 70% (95% CI 59-78%) from 28-34 days, then plateauing. From 14 days after the second dose a vaccine effectiveness of 89% (95%CI: 85-93%) was observed.

Individuals aged ≥70 years vaccinated from 4 January had a similar underlying risk of COVID-19 to unvaccinated individuals. With BNT162b2, vaccine effectiveness reached 61% (95%CI 51-69%) from 28-34 days after vaccination, then plateaued. With the ChAdOx1 vaccine, vaccine effects were seen from 14-20 days after vaccination reaching an effectiveness of 60% (95%CI 41-73%) from 28-34 days and further increasing to 73% (95%CI 27-90%) from day 35 onwards.

On top of the protection against symptomatic disease, cases who had been vaccinated with one dose of BNT162b2 had an additional 43% (95%CI 33-52%) lower risk of emergency hospitalisation and an additional 51% (95%CI 37-62%) lower risk of death. Cases who had been vaccinated with one dose of ChAdOx1 had an additional 37% (95% CI 3-59%) lower risk of emergency hospitalisation. There was insufficient follow-up to assess the effect of ChAdOx1 on mortality due to the later rollout of the vaccine. Combined with the effect against symptomatic disease, this indicates that a single dose of either vaccine is approximately 80% effective at preventing hospitalisation and a single dose of BNT162b2 is 85% effective at preventing death with COVID-19.

CONCLUSION: Vaccination with either a single dose of BNT162b2 or ChAdOx1 COVID-19 vaccination was associated with a significant reduction in symptomatic SARS-CoV2 positive cases in older adults, with even greater protection against severe disease. 

    Level 5

Dagan et al (2021) BNT162b2 mRNA COVID-19 Vaccine in a Nationwide Mass Vaccination Setting15

Data from Israel’s largest health care organization were used to evaluate the effectiveness of the BNT162b2 mRNA vaccine.

Methods:All persons who were newly vaccinated during the period from December 20, 2020, to February 1, 2021, were matched to unvaccinated controls in a 1:1 ratio according to demographic and clinical characteristics. Study outcomes included documented infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), symptomatic COVID-19, COVID-19-related hospitalization, severe illness, and death. The authors estimated vaccine effectiveness for each outcome as one minus the risk ratio, using the Kaplan-Meier estimator.

Results:Each study group included 596,618 persons. Estimated vaccine effectiveness for the study outcomes at days 14 through 20 after the first dose and at 7 or more days after the second dose was as follows: for documented infection, 46% (95% confidence interval [CI], 40 to 51) and 92% (95% CI, 88 to 95); for symptomatic COVID-19, 57% (95% CI, 50 to 63) and 94% (95% CI, 87 to 98); for hospitalization, 74% (95% CI, 56 to 86) and 87% (95% CI, 55 to 100); and for severe disease, 62% (95% CI, 39 to 80) and 92% (95% CI, 75 to 100), respectively. Estimated effectiveness in preventing death from COVID-19 was 72% (95% CI, 19 to 100) for days 14 through 20 after the first dose. Estimated effectiveness in specific subpopulations assessed for documented infection and symptomatic COVID-19 was consistent across age groups, with potentially slightly lower effectiveness in persons with multiple coexisting conditions.

    Level 5

Egunsola et al (2021) Transmissibility of COVID-19 among vaccinated individuals16

Objectives: To identify observational studies and randomized controlled trials evaluating the effectiveness of COVID-19 vaccination in reducing forward transmission from vaccinated people. The question has arisen because most COVID-19 vaccine trials use an endpoint of symptomatic infection, so there is less data around whether asymptomatic infection and viral carriage may still occur after vaccination, and whether this incurs a risk for viral transmission from vaccinated persons. 

Method: A targeted literature search of Clinicaltrials.gov, McMaster Health Forum (COVID-END), MedRxiv, Google, regulatory submissions, and websites of the Centres for Disease Control and Prevention (CDC) and World Health Organization (WHO) was conducted to identify RCTs or observational studies evaluating the effectiveness of COVID-19 vaccination in the prevention of asymptomatic infections and transmissibility of COVID-19 among vaccinated persons. 

Results: A total of 15 studies were included: 10 studies in humans, and 5 preclinical animal studies in macaques. 

Asymptomatic infection data were presented for only one component of the ChAdOx1 nCoV-19 vaccine studies. Participants were assessed by weekly self-administered nose and throat swabs for RT-PCR testing, with positive tests obtained regardless of symptoms. Neither one or two doses of AstraZeneca’s standard dose ChAdOx1 nCoV-19 vaccine was effective against asymptomatic infection with the wild type virus (7.8% (95% CI: -46.7-42.1) and 27.3% (95% CI: -17-54.9)) or B.1.1.7 variant (26.5% (95% CI: -112-74.5)) in baseline seronegative participants. The baseline PCR results of the participants were not reported. Therefore, persistent carriage after previous infection was not ruled out. In the subgroup of participants with an initial low dose of the vaccine, followed by a standard dose, two studies reported 49.3% (95% CI: 7.4-72.2) and 58.9% (95% CI: 1-82.9) respective efficacies against asymptomatic infection 14 days after the second dose. 

A phase 2/3 RCT comparing the minimum cycle threshold (Ct) values in ChAdOx1 nCoV-19 vaccinated individuals with a comparator group of meningococcal vaccine reported statistically significantly higher PCR Ct values in the ChAdOx1 nCoV-19 vaccinated group. 

Of the four Pfizer BioNTech’s BNT162b2 observational studies with data on asymptomatic infection, only one, involving almost 1.2 million Israeli participants, reported vaccine efficacy against asymptomatic infection. This study, which did not establish baseline seronegativity, found one dose of BNT162b2 to significantly reduce asymptomatic infection by 29% (95% CI: 17-39) and 52% (95% CI: 41-60) after 14 to 20 days and 21 to 27 days of follow-up respectively, as assessed by confirmed positive PCR SARS-CoV-2 test without documented symptoms. No routine swabbing was documented for the participants. The vaccine was reported to be 90% effective (95% CI: 83-94) against asymptomatic infection seven days after the second dose. 

The other three BNT162b2 studies did not report effect estimates for vaccine protection against asymptomatic infection. Two of these studies, however, measured Ct for BNT162b2 vaccinated and unvaccinated individuals; one of which found significantly higher Ct values (suggesting lower amounts of virus detected) in infected vaccinated individuals between 12 and 28 days after the first dose. In the other study, an asymptomatic screening program among health care workers (HCWs) who were vaccinated with one dose of BNT162b2, the median Ct values of infected health care workers were reported to have shown a nonsignificant trend towards increase between unvaccinated (Median=20.3) and vaccinated HCWs after 12 days post-vaccination (Median=30.3), suggesting that samples from infected vaccinated individuals had lower viral loads.

One study showed that, among participants who received the first dose of the mRNA-1273 vaccine while negative for COVID-19 by RT-PCR or antibody testing at baseline, 0.1% were found to have positive swabs but no symptoms at the time of their second dose, compared with 0.27% of the unvaccinated group. 

Among participants who were seronegative at baseline (defined as negative RT-PCR and negative serology against SARS-CoV-2 nucleocapsid on day 1), the Ad26.COV2.S vaccine was not effective against asymptomatic infection in the first 28 days of follow-up. However, the vaccine was 74% effective after 28 days. Asymptomatic infection was assessed by lack of symptoms on the day preceding, the day of, or any time after a positive PCR test. The frequency of swabbing for PCR testing was not reported. All of the animal studies showed the vaccines to significantly reduce viral load in vaccinated Ad26.COV2.S animals compared with controls. Viral load in fully vaccinated animals was assessed in bronchoalveolar lavage and nasal swabs between one and seven days after viral challenge.

Conclusion: Some studies, such as the ChAdOx1 nCoV-19 vaccine studies, included data on cross-sectional prevalence of positive SARS-CoV-2 RT-PCR from routine swabbing, which suggest effectiveness against asymptomatic infection, although this was not routinely assessed in a comparable way across studies. Limited evidence regarding the Ct values for AstraZeneca’s ChAdOx1 nCoV-19 vaccine and the BNT162b2 vaccine suggest their potential to reduce viral load and possibly transmission. 

    Level 5

Harris et al (2021) [Preprint] Impact of vaccination on household transmission of SARS-COV-2 in England17

Vaccination against SARS-CoV-2 with either ChAdOx1 nCoV-19 (AstraZeneca) or BNT162b2 (Pfizer-BioNTech) has been shown to produce a robust antibody response, and is effective in both preventing cases and reducing the severity of COVID-19 in vaccinated individuals. While fewer cases will reduce disease burden, it is not yet clear whether these vaccinations will also reduce transmission in the minority who have been vaccinated but develop post-vaccination infection. As a high risk setting for transmission, households can provide early evidence for impacts of interventions such as vaccines in preventing the onward transmission from an index case to household contacts. 

The authors compared household contacts of index cases receiving either the ChAdOx1 nCoV-19 or BNT162b2 vaccines with contacts of unvaccinated index cases, and the proportion of contacts who tested positive within 2-14 days of the index case (secondary cases) in each group: unvaccinated (base group), BNT162b2, and ChAdOx1 nCoV-19. 

In households where the index case was not vaccinated before testing positive, there were 96,898 secondary cases out of 960,765 household contacts (10.1%). There were 196 secondary cases in 3,424 contacts (5.72%) where the index case received the ChAdOx1 nCoV-19 vaccine 21 days or more before testing positive, and 371 secondary cases in 5,939 contacts (6.25%) where the index case received the BNT162b2 vaccine 21 days or more before testing positive. The unadjusted odds ratio for being a secondary case if the index case was vaccinated with ChAdOx1 nCoV-19 21 days or more before testing positive (vs. index case not vaccinated) was 0.55 (95% CI 0.46, 0.67), and for BNT162b2, 0.57 (95% CI 0.49, 0.65). Results from the multivariable model were similar, with an adjusted OR of 0.53 (95% CI 0.43, 0.63) for ChAdOx1 nCoV-19 and 0.51 (95% CI 0.44, 0.59) for BNT162b2. 

In the matched case-control study, 1,513 contacts of index cases vaccinated with ChAdOx1 nCoV-19 (64%) were matched to contacts with unvaccinated index cases, with an estimated OR of infection of 0.62 (95% CI 0.48, 0.79). There were 2,694 contacts of index cases vaccinated with BNT162b2 (67%) matched to contacts with unvaccinated index cases, with an estimated OR of 0.51 (95% CI 0.42, 0.62). 

    Level 7

Levine-Tiefenbrun et al (2021) [Preprint] Decreased SARS-CoV-2 viral load following vaccination18

Analyzing positive SARS-CoV-2 test results following inoculation with the BNT162b2 mRNA vaccine, the authors found that viral load is reduced 4-fold for infections occurring 12 to 28 days after first dose of vaccine. 

    Level 7

Milman et al (2021) [Preprint] SARS-CoV-2 infection risk among unvaccinated is negatively associated with community-level vaccination rates19

Mass vaccination has the potential to curb the current COVID-19 pandemic by protecting vaccinated individuals from the disease and possibly lowering the chance of transmission to unvaccinated individuals. The high effectiveness of the widely-administered BNT162b vaccine in preventing not only the disease but also infection suggests a potential for a population-level effect, critical for disease eradication. However, the effect is difficult to observe, especially in light of highly fluctuating spatio-temporal epidemic dynamics. Here, analyzing vaccination records and test results collected during a rapid vaccine rollout for a large population from 223 geographically defined communities, the authors find that the rates of vaccination in each community are highly correlated with a later decline in infections among a cohort of unvaccinated persons ≤16 years of age. These results provide observational evidence that vaccination not only protects individual vaccines, but also provides cross-protection to unvaccinated individuals in the community.

    Level 8 UNCLASSIFIED

Lipsitch and Kahn (2021) [Preprint] Interpreting vaccine efficacy trial results for infection and transmission20

Randomized controlled trials have shown high efficacy of multiple vaccines against SARS-CoV-2 disease (COVID-19), but there remains a paucity of evidence about vaccines’ efficacy against infection with and ability to transmit the virus. The authors describe an approach to estimate these vaccines’ effects on viral positivity, a prevalence measure which under reasonable assumptions forms a lower bound on efficacy against transmission. Specifically, separate analysis of positive tests triggered by symptoms (usually the primary outcome) and cross-sectional prevalence of positive tests obtained regardless of symptoms is recommended. The odds ratio of carriage for vaccine vs. placebo provides an unbiased estimate of vaccine effectiveness against viral positivity, under certain assumptions, and the authors show through simulations that potential departures from these assumptions will only modestly bias the estimate. Applying their approach to published data from the RCT of the Moderna vaccine, the authors estimate that one dose of vaccine reduces the potential for transmission by at least 61%.

    Level 8 UNCLASSIFIED

Petter et al (2021) [Preprint] Initial real world evidence for lower viral load of individuals who have been vaccinated by BNT162b221

One of the key questions regarding COVID-19 vaccines is whether they can reduce viral shedding. To date, Israel vaccinated substantial parts of the adult population, which enables extracting real world signals. The vaccination rollout started on December 20, 2020, utilized mainly the BNT162b2 Pfizer BioNTech vaccine, and focused on individuals who are 60 years or older. 

In this laboratory analysis, the authors traced the Ct value distribution of 16,297 positive qPCR tests between December 1 and January 31 in both ≥60 and 40-59 age groups. The authors hypothesized that if vaccines reduce viral load, a difference should be observed in the Ct values between the two age groups in late January, but not before. Consistent with this hypothesis, no statistically significant difference was found in the average Ct value between the age groups until January 15. In contrast, results in the last two weeks of January showed a significant weakening in the average Ct value of the ≥60 age group compared with the 40-60 age group. To further corroborate these results, the authors used a series nested linear models to explain the Ct values of the positive tests. This analysis favoured a model that included an interaction between age and the late January time period, consistent with the effect of vaccination. Demographic data and the daily vaccination rates were then used to estimate the effect of vaccination on viral load reduction. The study estimate suggests that vaccination reduces the viral load by 1.6◊ to 20◊ in individuals who are positive for SARS-CoV-2. This estimate might improve after more individuals receive the second dose. Taken together, the authors’ findings indicate that vaccination is not only important for the protection of individuals, but may also reduce transmission.

    Level 8 UNCLASSIFIED

Shah et al (2021) [Preprint] Effect of Vaccination on Transmission of COVID-19: an observational study in healthcare workers and their households22

The effect of vaccination for COVID-19 on onward transmission is unknown.

METHODS: A national record linkage study determined documented COVID-19 cases and hospitalisations in unvaccinated household members of vaccinated and unvaccinated healthcare workers from 8 December 2020 to 3 March 2021. The primary endpoint was COVID-19 14 days following the first dose.

RESULTS: The cohort comprised of 194,362 household members (mean age 31·1 ± 20·9 years) and 144,525 healthcare workers (mean age 44·4 ± 11·4 years). 113,253 (78·3%) of healthcare workers received at least one dose of the BNT162b2 mRNA or ChAdOx1 nCoV-19 vaccine and 36,227 (25·1%) received a second dose. There were 3,123 and 4,343 documented COVID-19 cases and 175 and 177 COVID-19 hospitalisations in household members of healthcare workers and healthcare workers respectively. Household members of vaccinated healthcare workers had a lower risk of COVID-19 compared to household members of unvaccinated healthcare workers (rate per 100 person-years 9·40 versus 5·93; HR 0·70, 95% CI 0·63 to 0·78). The effect size for COVID-19 hospitalisation was similar, with the confidence interval crossing the null (HR 0·77 (95% CI 0·53 to 1·10)). The rate per 100 person years was lower in vaccinated compared to unvaccinated healthcare workers for documented (20·13 versus 8·51; HR 0·45 (95% CI 0·42 to 0·49)) and hospitalized COVID-19 (0·97 versus 0·14; HR 0·16 (95% CI 0·09 to 0·27)). Compared to the period before the first dose, the risk of documented COVID-19 case was lower at ≥ 14 days after the second dose for household members (HR 0·46 (95% CI 0·30to 0·70)) and healthcare workers (HR 0·08 (95% CI 0·04 to 0·17)).

INTERPRETATION: Vaccination of healthcare workers was associated with a substantial reduction in COVID-19 cases in household contacts consistent with an effect of vaccination on transmission.


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Produced by the members of the National Health Library and Knowledge Service Evidence Team. Current as at 02 April 2021. This evidence summary collates the best available evidence at the time of writing and does not replace clinical judgement or guidance. Emerging literature or subsequent developments in respect of COVID-19 may require amendment to the information or sources listed in the document.  Although all reasonable care has been taken in the compilation of content, the National Health Library and Knowledge Service Evidence Team makes no representations or warranties expressed or implied as to the accuracy or suitability of the information or sources listed in the document.  This evidence summary is the property of the National Health Library and Knowledge Service and subsequent re-use or distribution in whole or in part should include acknowledgement of the service. 

Brendan Leen, Area Library Manager, HSE South [Author, Editor]; Melanie Surkau, Librarian, University Hospital Kerry [Author]; NIAC Subgroup Contributors: Dr. Peter O’Reilly; Dr. Niamh Bambury; Dr. Geraldine Casey; Dr. Kenneth Beatty; Dr. Paul Mullane; Dr. Philippa White.

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