Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention (CDC), and top infectious disease expert Dr. Anthony Fauci, testify before the Senate Health, Education, Labor, and Pensions Committee on Capitol Hill in Washington, Tuesday, July 20, 2021. Credit: J. Scott Applewhite / AP

The delta strain of the coronavirus spread widely in a series of densely packed summer events and large public gatherings in Cape Cod, Massachusetts, in early July, raising further concerns about the highly transmissible variant.

The findings show just how difficult the strain will be to control as Americans return to normal activities including parties and big gatherings. In July, 469 cases of COVID-19, primarily caused by delta, were recorded during the Barnstable County outbreak, and 74 percent of the cases occurred in fully vaccinated individuals, the Centers for Disease Control and Prevention said Friday in a study.

The findings contributed to the CDC’s recommendation that fully vaccinated people return to wearing masks indoors in areas of substantial and high transmission. Infections are soaring across the U.S., and the federal government and some localities are mandating masks and in some cases vaccines for workers.

The CDC abruptly relaxed its guidelines on masks in May, citing growing evidence that vaccines are effective against variants and that fully vaccinated people were at low risk to spread the virus to someone else. But as the delta variant made its way to the U.S., public-health experts urged the agency to recommend that even fully vaccinated people wear face coverings in public.

Anthony Fauci, the nation’s top infectious-disease expert, warned Sunday that the U.S. was moving in the wrong direction in combating delta. Former surgeon general Jerome Adams said the CDC acted prematurely when it eased its guidance on masks.

Masks for many people remain a potent symbol of the darkest days of the pandemic, and became political flashpoints in an election year shaped by the virus. Despite the delta variant fueling a surge in COVID-19 cases, states such as Arkansas, Iowa, Texas and Florida have prevented local governments from imposing mask mandates.

In the Massachusetts outbreak, almost four out of five vaccinated patients with breakthrough infections were symptomatic and four vaccinated patients were hospitalized. The findings were published in CDC’s Morbidity and Mortality Weekly Report. The new data show that the delta infection resulted in similarly high SARS-CoV-2 viral loads in vaccinated and unvaccinated people, CDC Director Rochelle Walensky said in a statement.

In the study, the CDC reported preliminary data suggesting people with breakthrough cases might have similar amounts of virus inside their bodies as unvaccinated people. In PCR diagnostic tests, a higher number of so-called “cycles” are needed to detect cases with lower levels of virus. But in the Cape Cod outbreak, there was no statistical difference in the number of PCR cycles needed to detect the virus between vaccinated and unvaccinated cases, suggesting the amounts of virus present in the patient samples wasn’t much different.

“High viral loads suggest an increased risk of transmission and [raises] concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus,” Walensky said in the statement. “This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation.”

The events took place among Massachusetts residents who had traveled to the Cape between July 3 and July 17. Many people with COVID-19 had been in crowded indoor and outdoor events at venues, including bars, restaurants, guest houses and rental homes.

While Provincetown wasn’t mentioned in the study, there were widespread reports of an outbreak at the popular vacation and party destination at the tip of Cape Cod.

When Pfizer Inc. and Moderna Inc. produced results late last year showing their vaccines could prevent more than 90 percent of symptomatic cases in the short term, the results were better than anyone expected.

But since then, it’s not only the virus that has changed with ever more transmissible variants: People’s behavior has changed as well, as more people attend parties and other large indoor social events where unmasked people might emit large quantities of virus. Those activities, along with variants, pose a much harder test for vaccines.

Story by Fiona Rutherford and Robert Langreth, Bloomberg News