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The vaccines may not prevent the virus but will protect against serious illness.

As vaccines for coronavirus disease 2019 (COVID-19) become available, you might be hearing stories of a small number of people who were vaccinated and still tested positive for the virus. This happened to a congressman from Massachusetts.

Fear not: This doesn’t mean the vaccines don’t work. Their job is to prevent serious illness and death, not mild or moderate cases. COVID-19 vaccines are an important step in helping end the pandemic. We are all ready to be able to hug loved ones, celebrate milestones and travel more freely.

Here’s what you should know about the vaccines.

1. The vaccines take time to work.

It takes about two weeks for your body to be able to fully fight the virus after receiving your second dose. Two of the vaccines available now require two doses that are given a few weeks apart. During the time between the doses and the three weeks after a second dose, you could still become infected. You could also pass the virus on to others as your body works on its ability to fight the virus.

You also might have been infected before you even got your shot. That infection can continue to develop after you get your shot, but before you are fully protected.

2. The vaccines protect against serious illness but they may not prevent infection.

The vaccines work to keep you from getting infection that leads to symptoms. They are very good at preventing illness that causes significant issues. This is similar to how people who get the flu shot may still get the flu, but a much milder case.  We don’t yet know if the COVID-19 vaccines prevent you from spreading the virus to others.

The good news is that even though “it is possible to be infected with the COVID-19 virus after you’ve been vaccinated, your chances of becoming seriously ill are much lower because you got the vaccine,” says UNC Health Chief of Infectious Diseases Joseph J. Eron Jr., MD.

3. The vaccines are very good—but not 100 percent effective.

None of the COVID-19 vaccines currently available are 100 percent effective. In fact, no vaccine is 100 percent effective for everyone because each person’s body reacts differently to vaccines.[1] In clinical trials, the Moderna and Pfizer vaccines kept about 9 out of 10 people safe, which is a very good result.

In clinical trials, Johnson & Johnson’s vaccine was 85 percent effective in preventing severe disease and essentially 100 percent effective in preventing hospitalization and death.

Even so, vaccine developers are looking at ways to make the vaccines even more effective. One way to do this is with booster shots. Booster shots are given after the initial vaccine to help improve or extend protection against the symptoms of the illness. They are common with vaccines, and you likely have gotten a booster vaccine for tetanus.

“Until we have more information, we have to assume that we have the potential to spread COVID-19 even if we’ve been vaccinated,” Dr. Eron says. “So that means we still have to take the precautions that we’ve talked about for so long: wearing a mask, washing our hands, and physical distancing so that we keep others safe.”


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