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Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021

Our take —

There was an outbreak of SARS-CoV-2 in Barnstable County, Massachusetts in which 469 cases were detected following a series of public events. A majority of the cases identified were among those who were vaccinated, and many of those experiencing breakthrough infections were symptomatic. The Delta variant made up 89% of the samples sequenced and cycle threshold (Ct) values were similar between those who were vaccinated and unvaccinated, suggesting similar viral loads. The results do not represent the overall rate of infection amongst all vaccinated and unvaccinated individuals attending events, but do speak to risks of breakthrough infection and potential transmission associated with the Delta variant even amongst fully vaccinated and suggest that additional non-pharmaceutical interventions including masks may be necessary for outbreak prevention.

Study design

Case Series

Study population and setting

In July 2021, an outbreak of COVID-19 was detected in Barnstable County, Massachusetts associated with multiple public events including gatherings drawing several individuals from out-of-state. A total of 469 cases were identified among Massachusetts residents, with further cases identified among residents of 22 other states not detailed in this report. Genomic sequencing was conducted (n=133) and cycle threshold (Ct) values were measured (n=211) in a subset of cases. Genomic sequencing can tell us about the specific variant of SARS-CoV-2 an individual was infected with, and Ct values can tell us how much virus an individual harbors in their nasopharynx (higher Ct values generally represent lower levels of virus). Breakthrough infections were defined as infections among those who were fully vaccinated (at least 14 days before exposure with 2 doses of either Pfizer BioNTech or Moderna or 1 dose of Janssen). For context, vaccination coverage among Massachusetts residents at the time was 69%.

Summary of Main Findings

Among the 469 cases identified, 346 or 74% were among those fully vaccinated (breakthrough cases). Of those 346 breakthrough cases, 274 or 79% experienced symptoms of COVID-19. Genomic sequencing conducted in 133 cases revealed that 89% (119/133) had the Delta variant (B.1.617). Median Ct values were similar among vaccinated (n=127) and unvaccinated, partially vaccinated, or vaccination status unknown (n=84) (22.77 vs. 21.54, respectively). Four out of the five hospitalized cases were among vaccinated individuals; no deaths were recorded.

Study Strengths

Inclusion of testing data, genomic sequencing, and Ct values helps provide a more complete picture of this outbreak among vaccinated and unvaccinated individuals.


This study is a case-series; vaccine coverage among all individuals attending public events during this time period is unknown. Thus, the high percentage of all cases among those vaccinated may have been influenced by: 1) the underlying level of vaccination coverage in an area (higher levels of vaccination coverage means more individuals are going to be vaccinated); and 2) detection bias (the sample is contingent on individuals seeking out testing). Comparisons of Ct values between those who were vaccinated and unvaccinated, in particular, should be interpreted in the context of this non-representative sampling approach; furthermore, the extent to which Ct values correspond to viral loads and viable replicable virus is not known in this setting. 

Value added

This report provides further information about SARS-CoV-2 among vaccinated individuals in a real-world setting. 

This review was posted on: 1 September 2021