There have been several highly publicized cases of people testing positive for the coronavirus even after receiving a COVID-19 vaccine.
Dr. Peter Chin-Hong, infectious disease expert with UCSF, says while these cases are in the minority, there are several ways this could happen and it does not mean that the vaccines do not work.
“The first possibility is they actually had COVID and were asymptomatic before getting the vaccine,” he explained.
After someone is infected it takes several days for the virus to multiply in their body before it can be detected on a test and develop symptoms, which means that even if everyone was tested before getting the vaccine, some positive results would still develop days later.
“The second, more likely possibility given the surge, is that after the vaccine you don’t get an immediate immune response. That’s why you need the second shot,” he said.
In California, some healthcare workers have just now started to receive their second doses of the Pfizer vaccine.
Clinical trials showed that the vaccine takes about 10 days after the first dose to provide protection, which increases significantly after patients get the second dose.
“The most likely possibility… is that they got exposed after the vaccine and the vaccine didn’t give them great immunity yet,” said Dr. Chin-Hong.
Dr. Chin-Hong says while the vaccine, like all safety measures, is not 100% effective, together they are highly effective at limiting the spread of the virus.