Was the large COVID-19 outbreak in Provincetown, MA, in July 2021 a superspreader event?

The question remains inconclusive, but genetic evidence in new preprint research strongly signals a superspreader type event, wherein a person or persons with high transmission capability — because of a variety of factors — results in infecting a majority of cases in an outbreak.

“We identified [at least] 40 distinct branches of the tree in our dataset that predate the outbreak, suggesting that the delta variant was introduced into this population from many sources,” the researchers report.1 “Six of these branches led to clusters (three or more cases) of varying sizes, of which one cluster — that comprised 83% (387/467) of outbreak-associated genomes — dominated this outbreak. The remaining five clusters each accounted for [less than] 4% of primary outbreak-associated cases.”

A “striking feature” of the dominant cluster was 158 (41%) identical consensus genomes, they reported.

“That suggests a single source, and is a signal that looks like superspreading,” said lead author Katherine Siddle, PhD, a postdoctoral fellow at Harvard and the Broad Institute. “It suggests transmission in a short period of time.”

The July 4, 2021, celebration in Provincetown extended over several days and at many different venues, and transmission likely was accelerated by poor weather that forced many events indoors. Critically, this was during the time period when the Centers for Disease Control and Prevention (CDC) said it was safe for vaccinated people to remain unmasked indoors. The third critical factor in this trifecta was that the highly transmissible delta variant was beginning to emerge rapidly.

The result of these multiple factors was that hundreds of fully vaccinated people had breakthrough infections, and as the genetic study confirms, transmitted the delta variant to other immunized people. The outbreak has been cited by anti-vaxxers as a sign the vaccines do not work, but only four immunized people were hospitalized, and none died. The CDC reinstated the mask recommendation for the vaccinated after the outbreak, as the delta variant quickly arose nationally to become the predominant strain of SARS-CoV-2.

However, the genetic findings were confounded by the outbreak epidemiology, since no one event or infected case could be confirmed as the source of the large cluster or the matching genomes at its genetic “root,” Siddle says.

“We talked to the health department [investigators] but we couldn’t link [the cluster] to a single person or a single event,” she says. “We can’t [definitively] say this was a superspreader event.”

The researchers concluded in the paper that “the genomic and epidemiological data taken together suggest that superspreading of the same viral sequence occurred at multiple locations. This is consistent with several scenarios, including one individual infecting others at multiple locations, or several individuals with the same virus, from either a common source or serial infection, transmitting independently.”

Coronaviruses a Common Superspreader

The superspreader phenomenon has been documented in other infectious diseases, including two other coronaviruses: the original severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS).

“During the 2003 SARS epidemic in Beijing, China, one hospitalized index patient was the source of four generations of transmission to 76 patients, visitors, and healthcare workers,” the authors of the review article on superspreading report.2 “During the MERS outbreak in South Korea, 166 (89%) of 186 confirmed primary cases did not further transmit the disease, but five patients led to 154 secondary cases. The index patient transmitted MERS to 28 other persons, and three of these secondary cases infected 84, 23, and seven [other people].”

The authors of the review also note that SARS-CoV-2 has caused many superspreader events. Some go further and argue it is the primary way the pandemic has spread globally.3 Certainly, crowded indoor settings are a setup for such transmission, but what makes an individual a superspreader biologically is the subject of some conjecture.

Theories include a higher viral titer, greater viral shedding, stronger emissions of aerosols when they speak or yell, and a tendency to get close to people unmasked when communicating. These conditions are not mutually exclusive, of course.

Still, Siddle emphasizes that those vaccinated in the Provincetown outbreak were following current CDC guidelines by not wearing masks at the indoor venues.

“We see transmission in this data both to and from vaccinated individuals,” Siddle says. “We weren’t able to quantify the extent to which that happened. We were not able to say that vaccinated people were equally likely to transmit [as the unvaccinated]. We didn’t have the data for that kind of analyses, but we definitely confirmed what was suspected at the time — transmission between vaccinated individuals.”

The genomic and epidemiologic data strongly support 25 transmission events, including many from the vaccinated to the vaccinated, Siddle and colleagues report. The genetic data alone shows evidence of an additional 64 transmissions. There appeared to be no significant difference between the outbreak-associated genomes and other publicly available delta variants.

“This was the first large outbreak among vaccinated people in the U.S. and it led to a return of masking, but it actually did not have a big downstream impact and I think that is pretty important to remember,” Siddle says.

One reason for that was many Provincetown events were targeted at the gay community, which certainly is experienced in public health incidents, and quickly communicated with others that an outbreak was underway.

“[They were] highly engaged and very proactive about contact tracing and alerting people who may have been exposed,” she says. “It really was very controlled, and it didn’t continue in Massachusetts or in other states.”

The researchers found that the genetic signature of the dominant cluster began decreasing after peaking at 9% of all delta genomes in the state on July 16. It was down to 4% by August 30 in Massachusetts. Cases were found in 37 other states after people left Provincetown. New York, California, and Georgia contained the largest number, with each representing approximately 10% of the 328 sequenced cases. The authors argue that this reflects rapid diminishment as the matching strain faded out.

“These findings suggest that, while the outbreak led to some onward transmission, it made at most a modest contribution to later delta cases within Massachusetts and a minimal contribution to cases elsewhere in the U.S.,” the researchers concluded. “The high rates of vaccination and the swift public health response, which included deployment of mobile testing, a local indoor masking mandate, and an extensive outreach campaign, likely contributed to the short duration and restricted impact of the outbreak.”


  1. Siddle KJ, Krasilnikova KA, Moreno GK, et al. Evidence of transmission from fully vaccinated individuals in a large outbreak of the SARS-CoV-2 delta variant in Provincetown, Massachusetts. medRxiv 2021; Oct 20. doi: 10.1101/2021.10.20.21265137. [Preprint].
  2. Frieden TR, Lee CT. Identifying and interrupting superspreading events—Implications for control of severe acute respiratory syndrome coronavirus 2. Emerg Infect Dis 2020;26:1059-1066.
  3. Lewis D. Superspreading drives the COVID pandemic — and could help to tame it. Nature 2021;590:544-546.