The scattered reports from around the country can play as a cruel irony: Someone tests positive for the coronavirus even though they’ve already received one or both doses of a Covid-19 vaccine.
Well-known examples
It has recently happened to at least three members of Congress:
But it has also been reported in people in other walks of life, including Rick Pitino, a Hall of Fame basketball coach and a California nurse.
How can that happen?
Experts say such cases are not surprising and do not indicate that there was anything wrong with the vaccines or how they were given. Here’s why.
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Vaccines don’t work right away. It takes a few weeks for the body to build up immunity after receiving a dose. And the vaccines now in use in the US, from Pfizer-BioNTech and Moderna, both need a few weeks after the first injection to be fully effective.
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Nor do they work retroactively. You may already be infected and not know when you will get the vaccine – even if you recently tested negative. That infection can continue to develop after you get the injection, but before the protection is fully set, and then show up in a positive test result.
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The vaccines prevent disease, but may not prevent infection. Covid vaccines are approved based on how well they prevent you from getting sick, needing hospitalization, and dying. Scientists don’t yet know how effective the vaccines are in preventing the coronavirus from infecting you in the beginning, or preventing you from passing it on to others. (That’s why vaccinated people should continue to wear masks and maintain social distance.)
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Even the best vaccines aren’t perfect. The efficacy rates for Pfizer-BioNTech and Moderna vaccines are extremely high, but they are not 100 percent. With the virus still getting out of control in the US, some of the millions of recently vaccinated people would become infected anyway.