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Breakthrough infections following COVID-19 vaccination were rare, most were mild, and the majority of those sequenced came from variant strains of the virus, CDC researchers found.

Of the 5% of cases sequenced, 64% were identified as variants of concern, such as the U.K. and California variant strains (B.1.1.7 and B.1.429, respectively), according to Marc Fischer, MD, and colleagues from the CDC COVID-19 Vaccine Breakthrough Case Investigations Team, writing in an early edition of the .

The team emphasized that more than 101 million people were vaccinated against COVID-19 from Jan. 1 to April 30, 2021, but "breakthrough cases are expected, especially before population immunity reaches sufficient levels to further decrease transmission."

By the end of April, there were 10,262 breakthrough infections reported across 46 U.S. states and territories. Median patient age was 58, and almost two-thirds occurred in women (63%). The authors said that based on preliminary data, 27% of these infections were asymptomatic.

Overall, 10% of cases were known to be hospitalized following breakthrough infection, and 2% of the patients died. Median age of patients who died was 82, and 18% were asymptomatic or died from a cause other than COVID-19.

"Vaccine breakthrough infections occur in only a small fraction of all vaccinated persons and account for a small percentage of all COVID-19 cases," the group wrote. "The number of COVID-19 cases, hospitalizations, and deaths that will be prevented among vaccinated persons will far exceed the number of vaccine breakthrough cases."

However, the investigators did note that the total reported cases among the fully vaccinated likely represents a "substantial undercount."

Of the 555 cases available for sequencing, the B.1.1.7 variant was responsible for more than half (56%) of cases; followed by B.1.429 in 25%; B.1.427, another California variant, in 8%; and P.1, the Brazilian variant, also in 8%.

Notably, the authors said, the proportion of cases due to variant strains was comparable to recent genomic surveillance data from March 28 to April 10, where variants of concern accounted for 70% of the weighted estimate of SARS-CoV-2 lineages submitted to CDC.

Starting May 1, the agency changed its monitoring of breakthrough infections to include only COVID-19 patients requiring hospitalization or who die.

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