Why Some Who Are Vaccinated Still Get Coronavirus

The scattered reports from around the country can play like a cruel irony: Someone tests positive for the coronavirus even though they have already received one or both doses of a Covid-19 vaccine.

It’s happened to at least three members of Congress recently:

But it’s been reported in people in other walks of life too, including Rick Pitino, a Hall of Fame basketball coach, and a nurse in California.

Experts say cases like these are not surprising and do not indicate that there was something wrong with the vaccines or how they were administered. Here is why.

  • Vaccines don’t work instantly. It takes a few weeks for the body to build up immunity after receiving a dose. And the vaccines now in use in the U.S., from Pfizer-BioNTech and Moderna, both require a second shot a few weeks after the first to reach full effectiveness.

  • Nor do they work retroactively. You can already be infected and not know it when you get the vaccine — even if you recently tested negative. That infection can continue to develop after you get the shot but before its protection fully takes hold, and then show