The scattered reports from across the country can play like a cruel irony: someone is testing the coronavirus positively, even though they have already received one or both doses of Covid-19 vaccine.
Striking examples
This has happened recently to at least three members of Congress:
But it has also been reported in people in other societies, including Rick Pitino, a Hall of Fame basketball coach, and a California nurse.
How can this happen?
Experts say cases like these are not surprising and do not indicate that there was anything wrong with the vaccines or how they were administered. Here’s why.
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Vaccines do not work immediately. It takes a few weeks for the body to build up immunity after receiving a dose. And the vaccines now used in the US, from Pfizer-BioNTech and Moderna, both require a second shot a few weeks after the first to reach full effectiveness.
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They also do not work retroactively. You may already be infected and do not know when you will get the vaccine, even if you have recently tested negative. The infection may continue to develop after you have been given the chance, but before its protection fully takes hold, and then appear in a positive test result.
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The vaccines prevent diseases, but perhaps not infection. Covid vaccines are approved based on how well they prevent you from getting sick, needing hospitalization and dying. Scientists do not yet know how effective the vaccines are in preventing the coronavirus from initially infecting you or preventing it from being transmitted to others. (Therefore, vaccinated people should continue to wear masks and maintain social distance.)
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Even the best vaccinations are not perfect. The effectiveness rates for Pfizer-BioNTech and Modern vaccines are extremely high, but they are not 100 percent. Since the virus in the US is still out of control, some of the millions of people recently vaccinated will become infected anyway.