Here's why some people test positive after getting a Covid-19 vaccine

A Covid-19 vaccine does not provide full or immediate protection, which means it's still possible to get infected and test positive for the virus.

Democratic Rep. Stephen Lynch of Massachusetts did. He tested positive after he got his second dose of the Pfizer vaccine. Hall of Fame basketball coach Rick Pitino, who coaches the men's team at Iona College in New York, also tested positive after getting his first dose.

They could have tested positive for a few reasons.

There's a lag between vaccination and protection

It takes a few days to a few weeks for the vaccine to work, according to the US Centres for Disease Control and Prevention. You could test positive before the vaccine kicked in.

"It takes a while for the immune response to develop," said Dr. Robert Salata, director of University Hospitals Roe Green Centre for Travel Medicine & Global Health in Cleveland.

The first dose may provide some protection, but as the Moderna CEO, Stephane Bancel said Monday "but we really just don't have any data to prove that at this point."

For Pfizer, after 14 days the first dose was around 52% effective at preventing disease, Salata, who was the lead investigator for the Pfizer vaccine at his hospital, said.

Vaccination prevents most, but not all disease

You could still test positive after being vaccinated since the vaccine is not 100% effective.

The two US-authorized vaccines are highly effective but they don't provide total protection.

The Pfizer vaccine was 95% effective at preventing illness in clinical trials after people got two doses.

The Moderna vaccine was 94% effective at preventing illness in people who got two doses in clinical trials.

Vaccination prevents disease, but infection, it's unclear

Vaccination prevents disease, but it's still unclear if, or how much, the vaccine prevents all infections.

"The information is less clear whether the vaccines will prevent the virus from infecting us and we can remain without symptoms. That's still under study." said Dr. William Schaffner, an infectious disease specialist and a professor of preventive medicine in the Department of Health Policy at Vanderbilt University.

"As far as what we've seen, these vaccines are really game changing at preventing disease and even severity of disease," said Namandje Bumpus, director of the department of pharmacology and molecular sciences at Johns Hopkins University.

"But focusing on the efficacy number doesn't paint the whole picture, because you could still end up with Covid, but by all indications that appears that those cases are still really less severe than unvaccinated people and that's really important."

The vaccine makers are still studying if the vaccines just keep people from getting really sick or if they totally protect from infection.

If you are asymptomatic you would still test positive for Covid-19. That would also mean even if you are vaccinated you could also still spread the disease. That's why even the vaccinated will still need to wear masks. A person could be an asymptomatic carrier and have the virus in their nasal passageways, so when they are breathing or speaking or sneezing they could still pass the novel coronavirus on to others.

Vaccines don't work retroactively

Vaccines don't work retroactively. You could test positive because you were infected before you got the vaccine and just did not know it yet. That's what happened to some of the health care workers in a study published by the US Centers for Disease Control and Prevention Monday.

The study found that 22 of 4,081 vaccinated health care workers tested positive for Covid-19 after getting their first dose.

One of the study authors, Dr. Eyal Leshem of Sheba Medical Center in Israel, said it was clear that some of the workers that tested positive "were actually infected with Covid before they get their first dose."

The variant question

There's concern that certain variants that have been spreading in the US could be less susceptible to the protection that comes from vaccines.

Preliminary lab data shows the vaccines should provide protection, and public health leaders want to get as many people vaccinated as quickly as possible to limit the opportunities the virus has to mutate.

The Covid-19 vaccine makers said they are testing to see if the vaccines work against the variants and they're also working on boosters that would add extra protection against variants.

"It is possible a year from now, that I'll get a flu shot in one arm and a Covid vaccine update booster in the other," Schaffner said. "We'll have to adapt ourselves to what it is that this virus is doing. And we have the capacity to keep up with the virus, and even get ahead of it."