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

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

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

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

Around 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 individuals in total — 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 wrote in the report.

The report also found that most breakthrough infections in the US — around 64% of cases, based on a sample of 555— were caused by variants of concern. That includes 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, 56%, while B.1.351 represented the least at just 4%. Meanwhile, 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 around 70% of all the coronavirus strains sequenced by the CDC. This alignment in numbers suggests that vaccines are protecting people from variants about as well as from the original strain.

Real-life studies also suggest that 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 a COVID-19 case caused by B.1.351 than unvaccinated people were. They were also around 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 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 tested positive for COVID-19 in April after being exposed to the virus through an unvaccinated coworker who wasn’t diligent about mask-wearing. 

“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.”

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