What is vaccine efficacy?

Simply put, vaccine efficacy is a measure of how well a vaccine works. It is calculated by comparing the percent of people who get COVID in the vaccinated group compared to the unvaccinated group. A perfect vaccine would completely prevent people from getting COVID, so vaccine efficacy would be 100%.

When is a new vaccine good enough?

How do we know if a new vaccine works well enough? Most Phase 3 trials are using vaccine efficacy as the main measurement to assess whether a vaccine works. If vaccine efficacy is 50% or more, scientists agree that it works well enough to go forward.

How does this translate to your risk of getting COVID?

Vaccine efficacy of 50% means that if you get vaccinated, you have a 50% reduced risk of getting sick, compared to your friend (who’s approximately the same age and similarly healthy) who didn’t get vaccinated.

Getting into the weeds

Although many people talk about vaccine efficacy as a single number, there are actually several different types of vaccine efficacy.

  • Efficacy to prevent infection: the virus doesn’t replicate in your body.
  • Efficacy to prevent disease: even if the virus invades your body, you don’t get sick.
  • Efficacy to prevent severe disease: even if you get sick, you don’t get super sick (like needing a ventilator).

Preventing infection entirely is the best kind of efficacy, but it’s the hardest to achieve. But a vaccine where people are still infected, but with less severe disease, is still really helpful.

According to Dr. Natalie Dean, a statistician and vaccine efficacy expert, most Phase 3 trials are measuring “efficacy to prevent disease” as the primary outcome of the clinical trials; “efficacy against infection” and “efficacy against severe disease” are secondary outcomes.

Last update: October 30, 2020, 4:00 pm ET

Science review: JAB