The Lancet
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).
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