Sample collection and case definitions

State and local authorities identified cases linked to the Provincetown outbreak using travel history and exposure data from the state COVID-19 surveillance system and follow-up case investigation. A primary outbreak-associated case was defined as receipt of a positive SARS-CoV-2 test result (nucleic acid amplification test or antigen test) ≤14 days after travel to or residence in Provincetown between July 3rd and July 17th, 2021. The majority of specimens in the present dataset were collected through mobile testing deployed in Provincetown by the MADPH following identification of the outbreak (Brown et al., 2021). Cases analyzed in this study ranged in age from 0 to 105 years old; 125 were female and 513 were male. Among cases with genome sequencing information, the age range was the same as for all patients and included 94 females and 373 males.

Cases that did not meet the above criteria as primary cases but were collected in Provincetown or had an epidemiologically confirmed link to a primary case (i.e., secondary cases) are collectively referred to here as “outbreak-associated.” COVID-19 vaccine breakthrough cases were defined on the basis of either i) documentation from the state immunization registry of completion of COVID-19 vaccination as recommended by the CDC Advisory Committee on Immunization Practices ≥14 days before specimen collection or ii) self-reported vaccination dose(s) indicating completion of COVID-19 vaccination ≥14 days before sample collection during follow-up case investigations. Individuals who had received at least one vaccine dose ≥1 day before sample collection but did not meet these criteria were defined as partially vaccinated.