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Vaccinated: What you need to know about breakthrough infections

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 September 1, 2021, more than 174 million people in the U.S. had been fully vaccinated. Of these, there were only 11,050 COVID-19 hospitalizations or deaths reported to the Centers for Disease Control and Prevention (CDC) by 49 states. (On May 1, CDC shifted to monitoring breakthrough cases only 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 observations 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.

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.

Additionally, certain individuals with a weakened immune system can now receive an additional dose, or “booster,” at least 28 days (4 weeks) after completing the initial two-dose vaccine. People with certain conditions that weaken the immune system are more likely to have a breakthrough infection than people who do not have compromised immune systems.

Those who are not immunocompromised are not immediately eligible for the booster, but the goal is for individuals to start receiving COVID-19 booster doses beginning in the fall, according to the CDC.

Check out CDC's latest advice on mask use and safe activities for fully vaccinated people at

Last Updated: 09/16/2021