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Why do some vaccinated people still get sick?

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April 30, 2021—There's no doubt that COVID-19 vaccines work. And all those shots going into arms? They are making a difference. But some fully vaccinated people have still gotten sick with COVID-19. How can that happen?

Experts call these "breakthrough cases," and they are not common. As of April 20, more than 87 million people in the U.S. had been fully vaccinated. Of these, only a small percentage of people still got COVID-19. There were 7,157 breakthrough cases during that time, according to the Centers for Disease Control and Prevention (CDC).

The fact that some breakthrough cases exist is not unusual, CDC notes. In fact, this occurs with all vaccines.

Clinical trials and real-world studies show that vaccines are very safe and highly effective. But no vaccine is 100% perfect. So even with a vaccine that prevents more than 90% of infections, a small number of people will still get sick.

Breakthrough cases can also happen for other reasons. For instance:

1. A person could be exposed to the virus just before or after they are vaccinated. It takes about two weeks after the final dose for the body to build immunity to the coronavirus. Only then are you considered fully vaccinated. If you're exposed during that time, you could still get sick.

2. Variants might account for some breakthrough cases. Some variants of the coronavirus may spread more easily. COVID-19 vaccines still seem to work against most of these variants. But it's possible some might be better at overcoming vaccine immunity.

CDC continues to look into reports of breakthrough cases. But so far, they don't seem to happen more often to people who share common risk factors.

Our best shot at beating COVID-19

Getting vaccinated will make you much less likely to get COVID-19. And if you are one of the few vaccinated people who still gets the disease? There is evidence that COVID-19 vaccines may make your illness less severe.

Vaccines are just one of our vital tools to end the pandemic. For this reason, CDC says you should still take steps to protect yourself and others. For instance, you should avoid large indoor gatherings. And you should wear a mask in public indoor spaces and crowded outdoor settings.

And there are times when you'll need to mask up if you're around unvaccinated people. Check out CDC's latest advice on mask use and safe activities for fully vaccinated people.

Want to learn more about COVID-19 vaccines? Visit our Coronavirus health topic center.

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