Most people with breakthrough infections experience mild illness, said Dr. William Moss, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health.
In the U.S., people who weren't vaccinated make up nearly all hospitalizations and deaths from COVID-19.
It's difficult to determine why any particular breakthrough case happens. How much of the virus you're exposed to could be a factor, Moss said. Our individual immune systems will also affect how well we respond to the shots. Some people, for example, have health conditions or take medications that could make their immune systems less responsive to the vaccines.
People might also have been exposed to the virus before the shots took full effect. Though less likely, they may have received a dose that was improperly stored or administered, Moss said.
The Centers for Disease Control and Prevention notes variants could be factors in some breakthrough cases, though evidence so far indicates the vaccines used in the U.S. are protective against them.
Health officials are also watching for signs that breakthrough cases are rising, which could signal that protection from the vaccines is fading and boosters are needed.
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