(HealthDay)—The delta variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) accounts for most cases of breakthrough infection among vaccinated individuals, according to research published in the July 30 early-release issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Catherine M. Brown, D.V.M., from the Massachusetts Department of Public Health in Boston, and colleagues describe an outbreak of 469 cases of COVID-19 associated with events and large public gatherings in a town in Barnstable County, Massachusetts. Among eligible Massachusetts residents, vaccination coverage was 69 percent.

The researchers found that 74 percent of the cases occurred in fully vaccinated individuals. The B.1.617.2 (delta) variant of SARS-CoV-2 was identified in 89 percent of the 133 patients who underwent genomic sequencing; the delta AY.3 sublineage was identified in one patient. Of the vaccinated patients with breakthrough infection, 79 percent were symptomatic. Five patients were hospitalized, four of whom were fully vaccinated; there were no deaths. Real-time reverse transcription-polymerase chain reaction cycle threshold values were similar in specimens from 127 vaccinated persons with breakthrough cases and in 84 persons who were unvaccinated, who were not fully vaccinated, or whose vaccination status was unknown.

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