The Centers for Disease Control and Prevention (CDC) released data Friday on the Cape Cod coronavirus outbreak that pushed the agency to update its mask guidance, showing that nearly three-quarters of confirmed cases were among the fully vaccinated.
The Massachusetts Department of Public Health’s analysis of COVID-19 samples from the outbreak determined that fully vaccinated patients carried similar amounts of the virus as unvaccinated patients.
The results from the mostly delta variant samples indicate that fully vaccinated people can potentially transmit and spread the strain, according to the CDC’s morbidity and mortality weekly report.
The state health department identified 469 COVID-19 cases among Massachusetts residents who went to Provincetown, a popular vacation destination in Barnstable County, in the month of July, including 346 fully vaccinated people.
Some 127 COVID-19 samples from the fully vaccinated, including recipients of all three U.S.-authorized vaccines, showed a similar viral load to the samples from the 84 unvaccinated people.
The report noted that microbiological studies are needed to confirm that similarity in the viral load to determine whether fully vaccinated people can transmit the virus.
The highly transmissible delta variant caused almost 90 percent of the 133 samples sequenced.
A total of 79 percent of the breakthrough cases were symptomatic. Five patients were hospitalized, four of whom were fully vaccinated, and no deaths were reported. Two of the fully vaccinated hospitalized patients had “underlying medical conditions.”
The lack of hospitalizations could suggest that the vaccines are in fact working but do not unilaterally stop all infections among the fully vaccinated.
The Massachusetts health department first was notified earlier this month that fully vaccinated and unvaccinated attendees of "densely packed" indoor and outdoor gatherings were testing positive for COVID-19. The gatherings included in bars, restaurants, guest houses and rental homes.
Provincetown's 14-day average COVID-19 case count rose from zero to 177 cases per 100,000 people by July 17.
CDC Director Rochelle WalenskyRochelle WalenskyFDA authorizes Pfizer COVID-19 vaccine booster shot for older and high-risk Americans Pfizer results offer hope amid worsening pandemic for children FDA panel endorses COVID-19 booster shots for older Americans, rejects widespread use MORE cited this data Tuesday when announcing the new mask recommendations for even fully vaccinated people to cover their faces in indoor public settings in counties with "substantial" and "high" COVID-19 transmission.
"This new science is worrisome and unfortunately warrants an update to our recommendations," Walensky said.
In a Friday statement, she said the research represents some of the data that influenced the updated mask guidance, noting that the agency is involved in many outbreak investigations across the country. She said the data from other outbreaks "will be rapidly shared with the public when available."
"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," Walensky said.
"This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation," she continued. "The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones. "
But the report suggests coronavirus prevention tactics could go beyond the updated mask recommendations.
“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,” it read.
The report emphasizes that its findings are "insufficient to draw conclusions about the effectiveness" of the vaccines against COVID-19 and the delta variant. It notes that with more vaccinated people among a particular population, vaccinated people are more likely to make up a higher percentage of COVID-19 cases.
Updated at 1:39 p.m.