• About three-fourths of people infected in a Massachusetts Covid-19 outbreak were fully vaccinated, according to new data published Friday by the CDC.
• The new data, published in the U.S. agency's Morbidity and Mortality Weekly Report, also found that fully vaccinated people who get infected carry as much of the virus in their nose as unvaccinated people.

About three-fourths of people infected in a Massachusetts Covid-19 outbreak were fully vaccinated against the coronavirus with four of them ending up in the hospital, according to new data published Friday by the Centers for Disease Control and Prevention.

The new data, published in the U.S. agency's Morbidity and Mortality Weekly Report, also found that fully vaccinated people who get infected carry as much of the virus in their nose as unvaccinated people, and could spread it to other individuals.

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

On Tuesday, the CDC reversed course on its prior guidance and recommended fully vaccinated Americans who live in areas with high Covid infection rates resume wearing face masks indoors. The guidelines cover about two-thirds of the U.S. population, according to a CNBC analysis.

While the delta variant continues to hit unvaccinated people the hardest, some vaccinated people could be carrying higher levels of the virus than previously understood and are potentially transmitting it to others, Walensky told reporters on a call Tuesday. She added the variant behaves "uniquely differently from past strains of the virus."

A CDC document that was reviewed by CNBC warned that the delta variant sweeping across the country is as contagious as chickenpox, has a longer transmission window than the original Covid strain and may make older people sicker, even if they've been fully vaccinated.

Delta, now in at least 132 countries and already the dominant form of the disease in the United States, is more transmissible than the common cold, the 1918 Spanish flu, smallpox, Ebola, MERS and SARS, according to the document. Only measles appears to spread faster than the variant.

The data published Friday was based on 469 cases of Covid associated with multiple summer events and large public gatherings held in July in Barnstable County, Massachusetts, which encompasses Cape Cod and is just outside Martha's Vineyard. The events were held in Provincetown, according to NBC News. Approximately three-quarters, or 74%, of the cases occurred in fully vaccinated people who had completed a two-dose course of the mRNA vaccines or received a single shot of Johnson & Johnson's.

Overall, 274 vaccinated patients with a breakthrough infection were symptomatic, according to the CDC. The most common side effects were cough, headache, sore throat, muscle pain and fever. Among five Covid patients who were hospitalized, four were fully vaccinated, according to the agency. No deaths were reported.

Testing identified the delta variant in 90% of specimens from 133 patients.

While numerous studies have shown that the vaccines don't work as well against the delta variant as they did against other strains, health officials say they are still highly effective, especially in protecting against severe illness and death. Roughly 97% of new hospitalizations and 99.5% of deaths in the U.S. are among unvaccinated individuals, U.S. health officials repeated this week.

The CDC also said the data has limitations. The agency noted that as population-level vaccination coverage increases, vaccinated persons are likely to represent a larger proportion of Covid cases. Additionally, asymptomatic breakthrough infections might be underrepresented because of detection bias, the agency said.

The CDC also said the report is "insufficient" to draw conclusions about the effectiveness of the authorized vaccines against Covid, including the delta variant, during this outbreak.