Efficacy of COVID-19 vaccines and why they are so difficult to compare

March 10 2021
By Ursula

With a number of COVID-19 vaccines on the market what does it mean if I’m vaccinated by one or the other vaccine? How will it affect my future freedom to travel, attend scientific meetings, party with friends? It would be great to know and great to know soon. But I won’t, according to the NYT / Nature comparison.

The NYT presents data from the Johnson & Johnson Adenovirus vector vaccine. Efficacy is defined as the percentage of people having been vaccinated not contracted COVID-19 compared to the people having been vaccinated with a placebo. Efficacy depends on the number of people having been vaccinated, the variants of SARS-CoV-2 present and the cutoff point of the outcome. For instance, while the J&J vaccine had a 72% efficacy in the US for 45‘000 people not to contract the disease, it was 85% effective for not contracting severe disease.

How confident can one be to trust these numbers? Different efficacy numbers can result from different clinical trials with for instance different age groups included or in the presence of different SARS-CoV-2 variants as well as the time point after vaccination at which efficacy is determined.

It will be the real world experience that will finally let us know about effectiveness, that is how much the vaccine reduces the risk of a disease out in the real world, in millions of people rather than thousands. As indicated in the Nature paper on “Vaccines are curbing COVID: Data from Israel show drop in infections”, it is not completely clear whether the drop in new infections is solely due to vaccination or whether it is also based on the nationwide lockdown.

What Do Vaccine Efficacy Numbers Actually Mean?

Why COVID vaccines are so difficult to compare

Vaccines are curbing COVID: Data from Israel show drop in infections