Source LANCET Open AccessPublished:April 20, 2021DOI:https://doi.org/10.1016/S2666-5247(21)00069-0
approximately 96 COVID-19 vaccines are at various stages of clinical development.1 At present, we have the interim results of four studies published in scientific journals (on the Pfizer–BioNTech BNT162b2 mRNA vaccine,2 the Moderna–US National Institutes of Health [NIH] mRNA-1273 vaccine,3 the AstraZeneca–Oxford ChAdOx1 nCov-19 vaccine,4 and the Gamaleya GamCovidVac [Sputnik V] vaccine)5 and three studies through the US Food and Drug Administration (FDA) briefing documents (on the Pfizer–BioNTech,6 Moderna–NIH,7 and Johnson & Johnson [J&J] Ad26.COV2.S vaccines).8 Furthermore, excerpts of these results have been widely communicated and debated through press releases and media, sometimes in misleading ways.9 Although attention has focused on vaccine efficacy and comparing the reduction of the number of symptomatic cases, fully understanding the efficacy and effectiveness of vaccines is less straightforward than it might seem. Depending on how the effect size is expressed, a quite different picture might emerge (figure; appendix).
Vaccine efficacy is generally reported as a relative risk reduction (RRR). It uses the relative risk (RR)—ie, the ratio of attack rates with and without a vaccine—which is expressed as 1–RR. Ranking by reported efficacy gives relative risk reductions of 95% for the Pfizer–BioNTech, 94% for the Moderna–NIH, 91% for the Gamaleya, 67% for the J&J, and 67% for the AstraZeneca–Oxford vaccines. However, RRR should be seen against the background risk of being infected and becoming ill with COVID-19, which varies between populations and over time. Although the RRR considers only participants who could benefit from the vaccine, the absolute risk reduction (ARR), which is the difference between attack rates with and without a vaccine, considers the whole population. ARRs tend to be ignored because they give a much less impressive effect size than RRRs: 1·3% for the AstraZeneca–Oxford, 1·2% for the Moderna–NIH, 1·2% for the J&J, 0·93% for the Gamaleya, and 0·84% for the Pfizer–BioNTech vaccines.