ChAdOx1 nCoV-19 vaccine: asymptomatic efficacy estimates – Authors’ response

It is well established that randomized controlled trials are the gold standard for estimating the effect of interventions. In such studies, the results of participants randomized to receive the new intervention are compared with those randomized to the control product, to produce unbiased estimates of the effect of the intervention.

Elizabeth McQuade and James Platts-Mills present a new analysis of published data from our randomized controlled trial
1
  • Voysey M
  • Costa Clemens SA
  • Madhi SA
  • et al.
Single Dose Administration and the Influence of Booster Dose Timing on the Immunogenicity and Efficacy of ChAdOx1 nCoV-19 Vaccine (AZD1222): A Pooled Analysis of Four Randomized Trials.

ChAdOx1 nCoV-19 vaccine (AZD1222) to further explore the effect of vaccination on preventing asymptomatic infections. McQuade and Platts-Mills propose a counterfactual which is the combination of symptomatic cases avoided and asymptomatic cases observed, to represent the hypothetical situation that could arise if a vaccine had only an effect on symptomatic infections and did not have the capacity to prevent asymptomatic infections. In this situation, an excess of asymptomatic cases in the vaccine group would be observed, which McQuade and Platts-Mills are trying to quantify.

Although the underlying assumptions may be too strict to be realistic, the results they showed are interesting and highlight the difficulties in estimating the true effectiveness of the vaccine against asymptomatic infections. We have shown efficacy against all cases, i.e. the combination of symptomatic or asymptomatic cases.
1
  • Voysey M
  • Costa Clemens SA
  • Madhi SA
  • et al.
Single Dose Administration and the Influence of Booster Dose Timing on the Immunogenicity and Efficacy of ChAdOx1 nCoV-19 Vaccine (AZD1222): A Pooled Analysis of Four Randomized Trials.

Analysis of any positive nucleic acid amplification test result maintains the benefits of trial randomization while including people who might have been symptomatic without vaccination. The very significant efficacy observed for this endpoint shows the overall benefit of the vaccine in preventing the opportunities for SARS-CoV-2 transmission.

1
  • Voysey M
  • Costa Clemens SA
  • Madhi SA
  • et al.
Single Dose Administration and the Influence of Booster Dose Timing on the Immunogenicity and Efficacy of ChAdOx1 nCoV-19 Vaccine (AZD1222): A Pooled Analysis of Four Randomized Trials.

The University of Oxford has partnered with AstraZeneca to further develop ChAdOx1 nCoV-19. AJP is chairman of the UK Department of Health and Welfare Joint Committee on Immunization and Immunization, but does not participate in discussions on COVID-19 vaccines, is a member of the WHO Strategic Advisory Group of Experts and from a UK National Institute for Health Research (NIHR) Principal Investigator. MV declares no competing interests. This research is funded by the NIHR, UK Research and Innovation, the Bill & Melinda Gates Foundation, the Lemann Foundation, Rede D’Or, the Brava and Telles Foundation and the South African Medical Research Council. We are grateful for the NIHR infrastructure provided by the NIHR Biomedical Research Centers and the NIHR Clinical Research Network at UK study sites. The views expressed in this correspondence are those of the authors and not necessarily those of NIHR or the UK Department of Health and Welfare.

Reference

  1. 1.
    • Voysey M
    • Costa Clemens SA
    • Madhi SA
    • et al.

    Single Dose Administration and the Influence of Booster Dose Timing on the Immunogenicity and Efficacy of ChAdOx1 nCoV-19 Vaccine (AZD1222): A Pooled Analysis of Four Randomized Trials.

    Lancet. 2021; 397: 881-891

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