Why Some Who Are Vaccinated Nonetheless Get Coronavirus

The scattered reports from across the country can seem like a cruel irony: someone tests positive for the coronavirus despite having already received one or both doses of a Covid-19 vaccine.

It recently happened to at least three members of Congress:

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

Experts say cases like this one aren’t surprising and don’t suggest anything was wrong with the vaccines or how they were given. Here’s why.

  • Vaccines don’t work right away. It takes a few weeks for the body to build immunity after a dose. And the Pfizer-BioNTech and Moderna vaccines currently in use in the US require a second shot a few weeks after the first to be fully effective.

  • They also do not work retrospectively. You may already be infected and not know when you get the vaccine – even if you recently tested negative. This infection can continue to develop after you get the shot, but before its protection is fully in effect and then shows up in a positive test result.

  • The vaccines prevent disease but may not prevent infection. Covid vaccines are approved based on how well they keep you from getting sick, hospitalized, and dying. Scientists don’t yet know how effective the vaccines are in preventing the coronavirus from infecting you from the start, or in preventing you from passing it on to others. (Therefore, vaccinated people should continue to wear masks and keep social distance.)

  • Even the best vaccines aren’t perfect. The efficacy rates for Pfizer BioNTech and Moderna vaccines are extremely high, but not 100 percent. With the virus still spreading out of control in the US, some of the millions of recently vaccinated people definitely had to get infected.

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