What does “95% vaccination efficacy”​ really mean?

Before COVID19, vaccine production and testing wasn’t something families discussed over the dinner table. But now, we have daily news alerts every time a pharmaceutical company passes a new hurdle towards vaccine authorization. One word we keep hearing over and over is efficacy, but what does a vaccine efficacy of 95% or 50% actually mean?


By Candela Iglesias Chiesa, MPH, PhD and Rochelle Caruso, MSc

When someone gets a vaccine with a 95% efficacy, we tend to interpret that as the person still having a 5% chance of getting COVID19. However, this is not an accurate interpretation. Efficacy is a comparison between groups of vaccinated and non-vaccinated people during a clinical trial, and thus, 95% efficacy means that you are 95% less likely to get COVID19 than someone who has not been vaccinated.

Another source of confusion around vaccine efficacy is the direct comparison between different vaccines. So it might seem that the Pfizer vaccine with 95% efficacy, is much better than the Johnson & Johnson vaccine with only 66% efficacy. However, we can’t directly compare the vaccine efficiencies of different products unless they were tested in parallel, in the same clinical trial, at the same time, in the same places and with the same populations.

While the J&J vaccine has a reported overall lower efficacy from its clinical trial than the Pfizer vaccine, the clinical trial settings were extremely different. The Pfizer vaccine was tested in the USA, while the J&J vaccine was tested in the USA, Latin American, and South Africa. J&J also included a more diverse participant base, both in terms of age, race, and people with underlying diseases and conditions.

The Pfizer clinical trial was completed before the emergence of viral variants. During the J&J trial, the B1.351 SARS CoV-2 variant, which was first identified in South Africa, began to emerge, which the J&J vaccine has a lower efficacy against. All of these factors can contribute to a lower overall efficacy in the clinical trial and prevent a direct comparison between the vaccines.

Therefore it doesn’t make sense for people to wait to get vaccinated until they can get a vaccine with supposed higher efficacy. If vaccines have passed all stringent authorization regulations, the best vaccine is the one that you can actually get.

A more important question to ask about any vaccine is: Will this vaccine keep me (or my loved ones) out of the hospital?Fortunately, for all the approved vaccines, the answer is yes. They have all shown VERY high efficacy against severe COVID19, hospitalization and death.

At the end of the day, any approved vaccine you can get access to is a great vaccine.

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