Reports scattered across the country can sound 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 happened to at least three members of Congress recently:
How can this happen?
Experts say cases like these are not surprising and do not indicate there was a problem with the vaccines or the way they were administered. Here’s why.
Vaccines do not work instantly. It takes a few weeks for the body to acquire immunity after receiving a dose. And the vaccines currently in use in the United States, from Pfizer-BioNTech and Moderna, both require a second injection a few weeks after the first to achieve full effectiveness.
They also do not operate retroactively. You may already be infected and not know it when you receive the vaccine – even if you have recently tested negative. This infection may continue to develop after you receive the vaccine, but before its protection fully sets in, and then show up in a positive test result.
Vaccines prevent disease, but maybe not infection. Covid vaccines are authorized based on their ability to prevent you from getting sick, needing hospitalization, and dying. Scientists don’t yet know how effective vaccines are at preventing the coronavirus from infecting you in the first place or at preventing you from passing it on to others. (That is why vaccinated people should continue to wear masks and maintain social distance.)
Even the best vaccines aren’t perfect. The efficacy rates of the Pfizer-BioNTech and Moderna vaccines are extremely high, but they are not 100%. With the virus still spreading uncontrollably in the United States, some of the millions of people recently vaccinated were likely to be infected anyway.