Pedestrians and cyclists head down Commercial Street in Provincetown, Mass., in May 2020. A study of a new outbreak in Provincetown found that three-quarters of those infected with the virus were fully vaccinated.

Pedestrians and cyclists head down Commercial Street in Provincetown, Mass., in May 2020. A study of a new outbreak in Provincetown found that three-quarters of those infected with the virus were fully vaccinated. / Getty Images

When revising its mask guidance this week to urge even vaccinated people to wear masks indoors in much of the country, the Centers for Disease Control and Prevention was criticized for not citing data in making that move.

Now it has — and the data is sobering.

The study details a COVID-19 outbreak that started July 3 in Provincetown, Mass., involving 469 cases. It found that three-quarters of cases occurred in fully vaccinated people. Massachusetts has a high rate of vaccination: about 69% among eligible adults in the state at the time of the study.

It also found no significant difference in the viral load present in the breakthrough infections occurring in fully vaccinated people and the other cases, suggesting the viral load of vaccinated and unvaccinated persons infected with the coronavirus is similar.

The CDC said the finding that fully vaccinated people could spread the virus was behind its move to change its mask guidance.

"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," Dr. Rochelle Walensky, the CDC's director, said in a statement Friday.

"This finding is concerning and was a pivotal discovery leading to CDC's updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones," Walensky said.

The good news is that the vaccines continue to be highly effective against the virus in preventing hospitalization and death from COVID-19. For instance, three studies from Canada, Singapore and Scotland have found that the Pfizer vaccine provides more than 90% protection against hospitalization and death.

Indeed, the Provincetown outbreak demonstrates the vaccines' effectiveness.

Alex Morse, town manager of Provincetown, said on Twitter that of the some 900 cases now linked to the cluster, "there have been no deaths, 7 hospitalizations, and the symptoms are largely mild."

"Our positivity peaked at 15% on 7/15 and was only 4.8% yesterday. The outbreak is contained and Provincetown is safe," Morse added.

As the delta variant continues to spread rapidly through the U.S., the CDC updated mask guidance now recommends people wear masks when inside public spaces in areas where the spread of the virus is "substantial" or "high," regardless of one's vaccination status.

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Tags: COVID-19  CDC  Series: The Coronavirus Crisis