Abstract and Introduction
During July 2021, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) B.1.617.2 variant infections, including vaccine breakthrough infections, occurred after large public gatherings in Provincetown, Massachusetts, USA, prompting a multistate investigation. Public health departments identified primary and secondary cases by using coronavirus disease surveillance data, case investigations, and contact tracing. A primary case was defined as SARS-CoV-2 detected ≤14 days after travel to or residence in Provincetown during July 3–17. A secondary case was defined as SARS-CoV-2 detected ≤14 days after close contact with a person who had a primary case but without travel to or residence in Provincetown during July 3–August 10. We identified 1,098 primary cases and 30 secondary cases associated with 26 primary cases among fully and non–fully vaccinated persons. Large gatherings can have widespread effects on SARS-CoV-2 transmission, and fully vaccinated persons should take precautions, such as masking, to prevent SARS-CoV-2 transmission, particularly during substantial or high transmission.
In recent months, the B.1.617.2 (Delta) variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread globally and has become the predominant circulating variant within the United States. Although the coronavirus disease (COVID-19) vaccines approved or authorized in the United States are highly effective,[2–4] including against the Delta variant,[5–7] several studies have indicated that variants of concern might be overrepresented among COVID-19 vaccine breakthrough infections[8,9] and that reverse transcription PCR cycle threshold values, which provide a crude correlation to the amount of virus in a sample, can be similar for vaccinated and unvaccinated persons infected with the Delta variant,[10,11] although viral load might decrease more rapidly among vaccinated persons. Studies before Delta variant predominance suggested that the risk for onward transmission from vaccinated persons to household members might be decreased compared with transmission from unvaccinated persons.[13,14] However, a more recent study during Delta predominance showed similar rates of household transmission from vaccinated and unvaccinated persons. In addition, previous studies of persons with COVID-19 vaccine breakthrough infections have indicated that illness might be more commonly asymptomatic or present with fewer symptoms than infections among non–fully vaccinated persons.[15,16]
In July 2021, after multiple, large public gatherings in Provincetown, Massachusetts, USA, a large outbreak of SARS-CoV-2 infections caused by the Delta variant was reported. Initial investigation by the Massachusetts Department of Public Health (MA DPH) identified 469 cases among Massachusetts residents during July 6–25; of these cases, 346 (74%) were characterized as COVID-19 vaccine breakthrough infections. We describe epidemiologic characteristics of the full multistate outbreak, document examples of secondary transmission, and assess whether illness differed by vaccination status.