Let’s Clear This Up: What Does 95% Covid-19 Vaccine Efficacy Actually Mean?

posted by Jason Kottke   Feb 18, 2021

In popular press and social media, there’s been a misunderstanding of what is actually meant when scientists say that the Pfizer and Moderna Covid-19 vaccines have an efficacy of 94-95%. It does not mean that 95% of vaccinated people are protected from infection — these vaccines are better than that. Dr. Piero Olliaro explains in a letter to The Lancet:

The mRNA-based Pfizer and Moderna vaccines were shown to have 94-95% efficacy in preventing symptomatic COVID-19, calculated as 100 x (1 minus the attack rate with vaccine divided by the attack rate with placebo). It means that in a population such as the one enrolled in the trials, with a cumulated COVID-19 attack rate over a period of 3 months of about 1% without a vaccine, we would expect roughly 0.05% of vaccinated people would get diseased.

Another way to put it: you’re 20 times less likely to get Covid-19 with a vaccine than without. (And again, data indicates these are safe vaccines.) Olliaro explains with some simple math:

If we vaccinated a population of 100,000 and protected 95% of them, that would leave 5000 individuals diseased over 3 months, which is almost the current overall COVID-19 case rate in the UK. Rather, a 95% vaccine efficacy means that instead of 1000 COVID-19 cases in a population of 100,000 without vaccine (from the placebo arm of the abovementioned trials, approximately 1% would be ill with COVID-19 and 99% would not) we would expect 50 cases (99.95% of the population is disease-free, at least for 3 months).

And of course if you vaccinate widely, it becomes a compounding situation because the virus just runs out of people to infect.