Anatomy of a Cape Cod Outbreak: How PIH’s Epidemic Intelligence Unit Supported a Major Public Health Investigation

A newly released report detailing a major outbreak of COVID-19 on Cape Cod, in Massachusetts, underscores the powerful transmissibility of the Delta variant and the way even fully vaccinated people who develop “breakthrough infections” can spread the virus. Indeed, 74% of infections in the Cape Cod cluster were in vaccinated individuals. 

“In July 2021,” the CDC report begins, “following multiple large public events in a Barnstable County, Massachusetts, town, 469 COVID-19 cases were identified among Massachusetts residents who had traveled to the town during July 3–17; 346 (74%) occurred in fully vaccinated persons. Testing identified the Delta variant in 90% of specimens from 133 patients.” 

It has now become clear that more than 800 cases have been linked to the outbreak. “The COVID-19 cluster in Provincetown that first emerged earlier this month has spiked,” according to The Boston Globe, and has impacted people across the United States. Still, it is important to note that even in this outbreak, “among persons with breakthrough infection, four (1.2%) were hospitalized, and no deaths were reported,” according to the report.  

In other words, COVID-19 vaccines, which are primarily intended to lower the risk of severe illness and death from the virus, are working. 

Still, the breadth and speed of the Cape Cod outbreak was a major driver in the CDC’s decision earlier this week to recommend that even fully vaccinated individuals should go back to wearing masks indoors in some regions where COVID-19 cases are surging. 

In the latest report, public health officials went further:  

“Findings from this investigation suggest that even jurisdictions without substantial or high COVID-19 transmission might consider expanding prevention strategies, including masking in indoor public settings regardless of vaccination status, given the potential risk of infection during attendance at large public gatherings that include travelers from many areas with differing levels of transmission,” the report concluded. 

The story behind the CDC report starts with public health sleuthing.

Soon after the Massachusetts Community Tracing Collaborative (CTC), a partnership between the Commonwealth and Partners In Health, launched in April 2020, a team of public health investigators known as the Epidemic Intelligence Unit was created to focus solely on “cluster-busting,” or pinpointing the sources of multiple infections, and then using a variety of tools to stop the spread. 

Throughout the pandemic, the unit has continually been on the lookout for clusters of infections, said Dr. John Welch, who leads the Massachusetts CTC, and earlier this month the team began hearing about cases linked to Cape Cod. At the same time, state health officials were also alerted about a spike in cases on the Cape. At that point, Welch said, the EIU was asked for support, and to focus, in particular, on business exposures.   

As part of its investigation, Welch said, the EIU made thousands of phone calls to every business impacted and to employees and customers – both known and potential contacts -- to get information to support local health departments in the investigation. The team also helped connect cases and contacts to resources, like food or other social support, they might need to safely isolate and quarantine, Welch added.  

He said that a major takeaway here is "the importance of a coordinated cluster response. We’re in this place now that if we can zero in on clusters, it can serve as the nexus of a more targeted COVID-19 response.” More generally, he said: “This should be a wake-up call for people who are eligible but remain unvaccinated. Vaccination is still the best tool to prevent serious illness, hospitalization, and death. We hope people use this moment to seek protection for their families and for communities around the country.” 

PIH’s Dr. Bram Wispelwey, senior technical lead for the U.S. Public Health Accompaniment Unit, which supports the COVID-19 response in hard-hit communities, said in a recent article that  “no vaccine is perfect, and breakthrough cases are expected; the Delta variant is causing more breakthrough cases than other versions of the virus. However, it is encouraging to see that the vast majority of these cases are either asymptomatic or mild. If you’re not vaccinated, you continue to be at significant risk of serious illness or death. While about half of Americans are unvaccinated, they currently make up at least 97% of hospitalizations and deaths.”

The New York Times, in its coverage of the new CDC report, reiterated the critical importance of vaccination and masking, now more than ever, and quoted Angela Rasmussen, a research scientist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, saying that: “Full vaccination is very protective, including against Delta. … Masks are a wise precaution, but the bulk of transmission is among the unvaccinated and that’s still who is most at risk.”