Outbreak of 469 symptomatic Delta cases in fully vaccinated adults during Bear Week

Simon Collins, HIV i-Base

On 30 July 2021, the US MMWR included a case study of outbreaks of symptomatic Delta infections associated with large public events in Provincetown, Massachusetts.

The average 14 day cases rose from an average of 0 before the events to a maximum of 177/100,000 within three weeks.

The case study is notably for several reasons.

  • 346/469 (74%) were in people who were fully vaccinated >14 days before being diagnosed. Of these, 46% had used Pfizer, 38% Moderna and 7% J&J – roughly similar to percentages in the local vaccination programme. Over half were residents.
  • Median time since end of 14-day post vaccine was 86 days (range: 6 to 178 days).
  • Nearly all 133 cases tested were the Delta variant (90%), with the remaining 10% not producing a result.
  • 274/469 (79%) were symptomatic. Asymptomatic infections were likely underestimated.
  • Only 4/469 (1.2%) were hospitalised – with 3/4 having received the single dose J&J vaccine. One additional unvaccinated person was also hospitalised. There were no deaths.
  • 85% were male – reflecting the demographics of the public events.

These data provide further evidence that currently approved vaccines are highly effective at preventing severe COVID-19.

Also, that continued use of masks and physical distances are important as restrictions are lifted.


The report is important for showing the high level of protection from hospitalisation against the Delta variant.

However, although the report referred to social events that were more likely to be attended by men they didn’t specify this was for two annual LGBT events in Provincetown Independence Week and Bear Week.

This might have increased the risk of transmission if people felt able to relax social mixing because of the high level of vaccination.

More than 900 cases have now been reported.


Brown CM et al. Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 2021. MMWR Morb Mortal Wkly Rep. ePub: DOI:10.15585/mmwr.mm7031e2. (30 July 2021).

This article was first posted on 1 August 2921.

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