Of the more than 130 million people in the United States who have been fully vaccinated against COVID-19, there have been reports of at least 10,262 breakthrough infections, according to the Centers for Disease Control and Prevention.
A breakthrough infection occurs when someone tests positive for coronavirus more than 14 days after the second dose of the Pfizer-BioNTech or Moderna vaccines or the single Johnson & Johnson shot.
Roughly a quarter of the breakthrough cases didn’t have symptoms — they were likely detected through routine testing, the CDC found in a report released Tuesday. Of 955 people who were hospitalized, about a third were in the hospital for reasons unrelated to COVID-19, or were asymptomatic. About 160 people, or 2%, died. Twenty eight of the deaths were unrelated to COVID-19. The CDC didn’t report whether the people had underlying conditions or comorbidities such as obesity.
The majority of the breakthrough infections were women, 63%, and a majority of the patients were 40 to 74 years old.
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The report cautions, however, that these cases are likely an underestimate because most people who have been fully vaccinated aren't being regularly tested. Recently, the CDC said that, with some exceptions, people who are fully vaccinated don’t need a coronavirus test, even if they’ve been exposed to the virus, unless they show symptoms.
On May 1, the CDC stopped regularly reporting on mild breakthrough infections so it could focus on investigating only cases severe enough to cause hospitalization or death. However, it's still working with local governments to understand whether variants are linked to infections after vaccination.
Through genomic sequencing, the CDC found that several variants of concern played a role in most of the breakthrough cases. The B.1.1.7 variant first identified in the U.K. was linked to 57% of cases, and a quarter were due to the B.1.429, first found in California. A small percentage of sequenced cases were caused by the P.1 from Brazil and the B.1.315 first identified in South Africa.
The proportion of cases due to variants of concern has also been similar to the proportion of these variants circulating throughout the country, the authors wrote.
It’s too soon to know which variants are more likely to result in breakthrough infections, but Dr. Carlos del Rio found the report “reassuring”.
“Breakthrough infections will happen as vaccines are not 100% effective,” del Rio, a professor of medicine at the Emory University School of Medicine, said in an email.
Del Rio took away a few things from the report: Many breakthrough infections were asymptomatic, many hospitalized patients were there for reasons unrelated to the virus, and overall there were very few deaths.
In the cases that tested positive after full vaccination, Del Rio added that he would have liked to see information about the cycle threshold of the cases, a value that provides information about the amount of virus that patients were carrying. He suspects many would likely not be infectious due to low levels of virus despite testing positive for COVID-19.
This story first appeared on NBCNews.com.