How many people have to get a SARS-CoV-2/COVID-19 vaccine to prevent a case of COVID-19? And a few ancillary points.
  • 170 cases of COVID-19
  • 162 in placebo group
  • 8 in vaccinated group
  • 10 severe cases of COVID-19; 9 in placebo, 1 in vaccinated group.
  • ~43,000 subjects, with 1:1 randomization, so ~21,500 people per group
  1. There isn’t all that much difference between needing to treat 140 people vs. needing to treat 124 people to see (or in this case, prevent) an outcome. Saying this vaccine doesn’t work when it works pretty close to as well as is literally theoretically possible is an unreasonable argument. The next step in the logical sequence would be that a vaccine isn’t necessary at all, but a vaccine has been the exit strategy for the pandemic since the start. There’s a very good reason for that: vaccines work.
  2. ARR and NNT are a function of the incidence of the phenomenon in question. By design, these trials stop at a relatively small number of cases, and in any given vaccine trial this will be rare, and if allowed to go for longer ARR would necessarily increase and NNT would drop.
  3. This is not how vaccine efficacy is computed.


I extend my sincerest gratitude to Dr. Rene Najera, who checked this piece for accuracy and made sure my reasoning did not become circular, even when it was very tempting. I must also thank Victoria Crabb MS, BSN for checking my epidemiology and her unwavering support.


1. A vaccine is the one true global exit strategy from this pandemic, but timeline is frustratingly long. 2020 Apr 7 [accessed 2020 Dec 3].



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Edward Nirenberg

Edward Nirenberg


I write about vaccines here. You can find me on Twitter @enirenberg and at (where I publish the same content without a paywall)