How well do the COVID vaccines work? (and why has this been so confusing?)

Kristen Panthagani, MD, PhD

So that was the bar: 50% efficacy. And that’s not just talking about efficacy against infection. Even if a vaccine didn’t reduce the risk of infection at all, but reduced the risk of severe disease or death by 50%, it would have met the FDA’s threshold for efficacy.

Isn’t 50% efficacy kind of low?

If you’re thinking this seems low compared to other vaccines, you’re both right and wrong. The flu vaccine, which is updated every year to match circulating flu strains, has an efficacy of 40-60%, so fairly similar to the minimum bar set by the FDA for COVID vaccines.  But if we compare to many of the vaccines we get in childhood, then 50% is quite a bit lower. The two dose MMR vaccine series is 97% effective against measles, and 88% effective against mumps. At least 3 doses of the polio vaccine is 99-100% effective against polio. That’s much higher than 50%. 

But if you consider that we were at the beginning of a global pandemic that was killing thousands of people every day, then reducing the risk of death by 50% would be a major win. It may seem low relative to the efficacy of many of our childhood vaccinations, but even a vaccine that reduced disease severity by half would save thousands of lives.

So the bar was set. And the scientists and doctors who were following vaccine development had their expectations managed: anything above 50% efficacy would be considered by the FDA, and if authorized, we would have a new major tool to help us fight SARS-CoV-2.