It is rare, but not unheard of, for people getting the Pfizer or Moderna coronavirus vaccines to contract covid-19 mid-vaccination — that is, between doses.

The two-dose mRNA vaccines, authorized for emergency use last year by the Food and Drug Administration, were shown in U.S. clinical trials to be about 95 percent effective at preventing infection among those who were fully vaccinated.

But the vaccines are less effective — about 80 percent — between the first and second doses, according to a real-world study from the Centers for Disease Control and Prevention. That means about 20 percent of those who are only partially vaccinated could still get infected.

Health experts say that does not mean people in that camp should skip the second dose.

Anthony S. Fauci, the nation’s leading infectious-diseases expert, said this month that people can get the second shot after recovering from the infection and meeting the criteria for discontinuing isolation.

How did I get covid-19 mid-vaccination?

Health experts said there are various reasons someone might contract covid-19, the illness caused by the coronavirus, between the two doses of the mRNA vaccines. But they said there is no scenario in which the vaccine itself would cause the infection.

Some people may simply feel safer after getting their first injection and let down their guards.

Others may live in a community with higher transmission rates and, therefore, be at increased risk.

Or it may just be the luck of the draw.

William Moss, a professor of epidemiology and immunology at the Johns Hopkins Bloomberg School of Public Health, said although the mRNA vaccines are considered highly effective, they “are not perfect and are less so after only a single dose, so people can still get infected.”

In one real-world study, the CDC reported that a small number of people contracted covid-19 while waiting for their second shot.

But Moss, executive director of Hopkins’s International Vaccine Access Center, said that even when partially vaccinated people get the virus, it is likely that the severity of the disease will be less for those who have received at least one dose — though researchers do not yet have the data to prove it.

So what do I do now?

Health experts said that people who contract covid-19 in the middle of their vaccine series should still get the second dose, because the immune response from natural infection is so variable. But depending on when in the process individuals get sick, they may need to postpone it.

The CDC recommends that people with mild to moderate infection isolate for at least 10 days after the onset of symptoms or until they have recovered and at least 24 hours after being fever-free.

So those who get covid-19 within the first week or so after their first dose may be fine to get the second dose on time, assuming they have complied with the recommendations. The second dose of the mRNA vaccines is supposed to be given 21 days after the first dose for Pfizer and 28 days for Moderna.

Those who start experiencing symptoms less than 10 days before the shot should reschedule.

Inci Yildirim, a pediatric-infectious-disease physician and vaccinologist at the Yale School of Medicine, said with most viral vaccines, including the coronavirus vaccines, it is not advisable to vaccinate people who have a fever or are otherwise ill. The reason is because medical professionals need to be able to differentiate between symptoms that are caused by an infection and those that are simply a side effect of the vaccine, she said.

Also, Yildirim said, people who are planning to be vaccinated should be well, because immune cells need to be healthy to have a robust reaction to vaccine.

That waiting period is also designed to protect others — such as in vaccination centers, for example — from exposure to the virus.

Experts said the only instances in which people are advised to wait longer than 10 days to get their shot is when they have a specific medical issue that needs to be managed or when they have had covid-19 and been treated with convalescent plasma or lab-manufactured monoclonal antibodies.

Some people on immunosuppressive therapies may need to delay a dose because the therapies can blunt the immune response to the vaccine, for instance.

And those who have received convalescent plasma or monoclonal antibodies are told to wait at least three months before getting the shot because they will have antibodies against the coronavirus spike protein “and the vaccine coming in will be spike-protein-induced protection,” Yildirim said.

Yildirim said that for those who already have the antibody against the spike protein, “we don’t know as of today how it will impact vaccine effectiveness.”

How can people guard against infection while building immunity?

It takes several weeks after vaccination to build immunity.

That’s why, experts said, it’s important to continue taking the recommended precautions — mask-wearing, social distancing and lots of hand-washing.

“Because we know that those measures are really helpful and they do work,” Yildirim said.

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