Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine breakthrough infections occur in a small percentage of vaccinated individuals, according to research published in the May 25 early-release issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Meseret Birhane, from the CDC in Atlanta, and colleagues examined SARS-CoV-2 infections among persons who are fully vaccinated and monitored trends in case characteristics and SARS-CoV-2 variants identified from people with these infections.

The researchers note that as of April 30, 2021, 10,262 SARS-CoV-2 vaccine breakthrough infections had been reported from 46 U.S. states and territories. Sixty-three percent of these cases occurred in women, with a median patient age of 58 years. Overall, 27 percent of vaccine breakthrough infections were asymptomatic, and 10 and 2 percent of patients were known to be hospitalized and died, respectively. Of the hospitalized patients, 29 percent were asymptomatic or hospitalized for an unrelated reason. The median age was 82 years for patients who died; 18 percent of those who died were asymptomatic or died from an unrelated cause. Of the sequence data available from 555 cases, 64 percent were identified as variants of concern, including B.1.1.7, B.1.429, B.1.427, P.1, and B.1.351 (56, 25, 8, 8, and 4 percent, respectively).

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