Transmission from vaccinated individuals in a large SARS-CoV-2 Delta variant outbreak

An outbreak of over one thousand COVID-19 cases in Provincetown, Massachusetts, in July 2021—the first large outbreak mostly in vaccinated individuals in the US—prompted a comprehensive public health response, motivating changes to national masking recommendations and raising questions about infection and transmission among vaccinated individuals. To address these questions, we combined genomic and epidemiological data from 467 individuals, including 40% of known outbreak-associated cases. The Delta variant accounted for 99% of outbreak-associated cases in this dataset; it was introduced from at least 40 sources, but 83% of cases derived from a single source, likely through transmission across multiple settings over a short time rather than a single event. Genomic and epidemiological data supported multiple transmissions of Delta from and between fully vaccinated individuals. However, despite its magnitude, the outbreak had limited onward impact in MA and the US, likely due to high vaccination rates and a robust public health response.
Rapid integration of epidemiological and genomic data helped to support public health measures during a recent large outbreak of the SARS-CoV-2 Delta variant. This outbreak stemmed primarily from a single source that spread the virus at multiple locations and frequent transmission between vaccinated individuals was observed and contained with robust intervention and community engagement.