Why do some vaccinated people still get COVID-19?

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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 Oct. 4, 2021, more than 185 million people in the U.S. had been fully vaccinated. Of these, around 16,900 were hospitalized with a COVID-19 breakthrough infection and around 6,600 died from one, according to the Centers for Disease Control and Prevention (CDC). (CDC only monitors breakthrough cases in which someone was hospitalized or died.)

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 account for some breakthrough cases. The Delta variant, for example, causes more infections and spreads faster than earlier strains of the virus. Vaccines are still very effective against severe illness, but breakthrough infections from Delta may be more likely. 

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.

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

Reviewed 10/5/2021

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