Breakthrough coronavirus cases after vaccines are very rare and mostly due to variants, a new CDC report found

• About 10,000 out of 100 million vaccinated Americans got COVID-19 after their shots, the CDC found.
• Most of these "breakthrough infections" were caused by variants of concern, such as B.1.1.7.
• But these infections were still rare and mostly mild - a sign that vaccines are holding up well.

By the end of April, more than 100 million Americans had received their coronavirus vaccines.

Clinical-trial data had indicated the vaccines would reduce the risk of getting symptomatic COVID-19 by about 66% to 95%, depending on which shot the person got. A new report from the Centers for Disease Control and Prevention suggests that was indeed the case.

Still, as of April 30, about 10,000 Americans had developed "breakthrough infections," or cases of COVID-19 diagnosed at least two weeks after they were fully vaccinated. That's a rate of about 0.01%.

About 27% of those infections were asymptomatic, meaning the vaccines performed as expected by preventing people from feeling sick. Another 10% of people with breakthrough infections were hospitalized (some for reasons other than COVID-19, though most were sick from the disease). And 2% of people with breakthrough infections - 160 people - died.

Overall, the new data indicates that breakthrough infections are extremely rare and mostly mild.

"Even though FDA-authorized vaccines are highly effective, breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission," the CDC report said.

The report also found that most breakthrough infections in the US - about 64% of cases, based on a sample of 555 - were caused by variants of concern: B.1.351 (the variant first identified in South Africa), B.1.1.7 (discovered in the UK), P.1 (found in Brazil), and two variants discovered in California, B.1.427 and B.1.429.

B.1.1.7 represented the majority of breakthrough infections, at 56%, while B.1.351 represented the fewest, at 4%. B.1.429 made up 25% of breakthrough infections, and B.1.427 and P.1 each represented 8%.

That 64% figure is pretty close to the CDC's estimate of the total share of US coronavirus cases caused by variants of concern. From March 28 to April 10, these variants represented about 70% of the coronavirus strains sequenced by the CDC. This alignment suggests that vaccines are protecting people from variants about as well as they are from the original strain.

Real-world studies have also suggested the vaccines are holding up well against variants.

New research found that people in Qatar who were fully vaccinated with Pfizer's shot were 75% less likely to get COVID-19 caused by the B.1.351 strain than unvaccinated people were. They were about 90% less likely to develop COVID-19 caused by B.1.1.7.

Even so, some people will get sick after their shots. Anna Kern, a 33-year-old nurse practitioner in Ferndale, Michigan, was among them.

"It feels weird to be a statistical anomaly," Kern told Insider.

Kern received her second dose of Pfizer's vaccine in January. She said she tested positive for COVID-19 in April after being exposed to the virus through an unvaccinated coworker who wasn't diligent about wearing a mask.

"I feel like those people who are getting COVID after being vaccinated for the most part are the people who have been really, really cautious for a really long time," Kern said. "So when you do get it, you feel lots of guilt - like, what did I do wrong? How could I have been more cautious?"

Kern said her primary symptoms were chills and fatigue, but she thinks the vaccine helped prevent a more severe outcome.

"I am still very grateful that I was vaccinated," she said. "I know this could have been a lot worse."