• A series of large public events in a Massachusetts town in July 2021 has been linked to 469 new COVID-19 cases. Three-quarters of these cases were in fully vaccinated people.
• The vast majority of infections were with the Delta variant of the virus.
• Fully vaccinated individuals who contracted the virus appeared to be just as infectious as those who had not been fully vaccinated.
• As a result, the Centers for Disease Control and Prevention (CDC) now recommend wearing masks in indoor public settings in areas with substantial or high transmission.

Over 2 weeks in July 2021, thousands of people from across Massachusetts gathered in Provincetown in Barnstable County to celebrate Independence Week, followed by Bear Week.

At the start of the festivities on July 3, the Massachusetts Department of Public Health reported that there had been a daily average of 0 cases of COVID-19 per 100,000 county residents during the previous 14 days.

By the end of the celebrations on July 17, however, that figure had increased to an average of 177 daily cases per 100,000 residents.

People with confirmed cases of COVID-19 reported that they had attended crowded indoor and outdoor events at venues including bars, restaurants, guest houses, and rental homes.

By July 26, a total of 469 cases had been confirmed. Of these, 346 (74%) were in fully vaccinated individuals. Overall, 274 (79%) vaccinated people who tested positive for COVID-19 were showing symptoms.

The Delta variant of the virus accounted for 90% of the samples that scientists genetically sequenced from 133 of the cases.

The cycle threshold value — which is an indirect measure of the concentration of virus particles in the body, or “viral load” — was similar for vaccinated and unvaccinated individuals. This suggests that even fully vaccinated people who contract the Delta variant are highly infectious.

In its report, the CDC concludes:

“Jurisdictions might consider expanded prevention strategies, including universal masking in indoor public settings, particularly for large public gatherings that include travelers from many areas with differing levels of SARS-CoV-2 transmission.”

In response to data from the outbreak in Provincetown, the CDC tightened its guidance for fully vaccinated individuals on July 27 to advise wearing a mask in indoor public places for people who are “in an area of substantial or high transmission.”

At the time of writing, the majority of counties in the United States fall into these categories.

“The masking recommendation was updated to ensure [that] the vaccinated public would not unknowingly transmit [the] virus to others, including their unvaccinated or immunocompromised loved ones,” said CDC director Rochelle Walensky in a statement.

Is herd immunity a mirage?

The outbreak overturned hopes that fully vaccinated individuals would not need to take any further precautions to prevent contracting the virus or transmitting it to others.

It also calls into question the possibility of herd immunity, which experts believe occurs when enough people have either been vaccinated or had an infection. In theory, this can break the chain of transmission and snuff out an outbreak of infectious disease.

According to the latest data from the CDC, 49.8% of the U.S. population has been fully vaccinated, and 58% of the population has received at least one dose.

In Massachusetts at the time of the outbreak, 69% of adults were fully vaccinated.

This strongly suggests that, at least for the highly transmissible Delta variant, herd immunity remains elusive.

Prof. Andrew Noymer, Ph.D., an epidemiologist at the University of California at Irvine, told the BMJ: “If the vaccinated can [contract the infection] and, we believe from other studies, potentially spread [SARS-CoV-2], then herd immunity becomes more mirage than oasis.”

Despite the findings from the outbreak in Provincetown, the vaccine does appear to provide good protection against severe disease.

The CDC reports that only 5 of the 469 COVID-19 cases led to hospitalization. Of these cases, four were in fully vaccinated individuals, of whom two had underlying medical conditions.

There had been no deaths arising from this outbreak by July 30, which was when the CDC published its report.

The latest data from the CDC suggest that while vaccinated people can develop symptomatic infections, known as “breakthrough cases,” hospitalizations and deaths are rare.

It reports that by July 26, more than 163 million people in the U.S. had been fully vaccinated against COVID-19. Of these, 6,239 have been hospitalized with the disease, and 1,263 have died. This equates to a death rate of 0.0008%.

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