Here's why some test positive for Covid-19 after being vaccinated

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.

Notable examples

It has happened to at least three members of Congress recently:

— Adriano Espaillat, D-N.Y.

— Stephen Lynch, D-Mass.

— Lori Trahan, D-Mass.

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

How can that happen?

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 United States, 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 up in a positive test result.

— The vaccines prevent illness, but maybe not infection. Covid vaccines are being authorized based on how well they keep you from getting sick, needing hospitalization and dying. Scientists don’t know yet how effective the vaccines are at preventing the coronavirus from infecting you to begin with, or at keeping you from passing it on to others. (That is why vaccinated people should keep wearing masks and maintaining social distance.)

— Even the best vaccines aren’t perfect. The efficacy rates for Pfizer-BioNTech and Moderna vaccines are extremely high, but they are not 100%. With the virus still spreading out of control in the United States, some of the millions of recently vaccinated people were bound to get infected in any case.