Vaccinated people can have the same amount of the virus as unvaccinated people.

On Friday, US health officials released details of the investigation, which was pivotal in the Centers for Disease Control and Prevention (CDC) decision this week to recommend that vaccinated people return masked indoors in areas of the US where the delta option exists. promotes the growth of infections.

The study authors even argue that the results indicate that the CDC’s guide to mask use should be expanded across the country, even for outdoor events.

The findings could change the way the disease is transmitted. It used to be thought that vaccinated infected people had low levels of the virus and should not pass it on to others. But new information shows that this is not the case for the delta option.

The outbreak in Provincetown – a resort in Cape Cod, the county with the highest vaccination rate in Massachusetts – has already affected more than 900 cases. Of these, about three quarters concern people who have been fully vaccinated.

Like many states, Massachusetts lifted all restrictions related to the new coronavirus in late May, ahead of Memorial Day, which marks the start of the summer season. But Provincetown this week reinstated the requirement for masks for all people.

Publicly released internal CDC documents on the outbreak and delta variant suggest that the CDC may be considering other changes to the U.S. coronavirus control guidelines, such as guidelines for widespread use of masks and requiring doctors and other healthcare professionals to be vaccinated.

The delta variant causes infections that are more contagious than regular flu, smallpox and Ebola, and no less contagious than chickenpox, according to documents that mention cases in Provincetown.

The documents were obtained from the Washington Post newspaper. As they note, vaccines against the new coronavirus are very effective against the delta variant in preventing death and serious illness.

The Provincetown outbreak and documents highlight the huge challenge the CDC faces in promoting vaccinations, while recognizing that outbreaks can occur, even if they are rare.

The documents are guidelines for public statements by CDC staff. One paragraph contains advice: “Become aware that the war has changed,” clearly referring to the growing concern that the many millions of vaccinated people could be the source of widespread infection.

An agency spokesman declined to comment.

While experts agree with the CDC’s revised position on indoor mask use, some say the Provincetown outbreak report does not prove vaccinated people are a significant source of new infections.

“Recommendation[CDC]has scientific credibility. But that doesn’t follow from this study, ”said Jennifer Nuzzo, a public health researcher at Johns Hopkins University.

The CDC study is based on an estimated 470 cases of infection associated with the holidays in Provincetown, which included large-scale events, indoor and outdoor, in bars, restaurants, rented homes and other living spaces.

Researchers conducted tests separately from this universe and found roughly the same levels of the virus in people who were vaccinated and in others.

Three quarters of infections were in fully vaccinated people. Among those who received all doses of the vaccine, about 80% experienced symptoms such as cough, headache, fever, sore throat, and muscle aches.

CDC executives said more research remains to be done and scaled up, tracking tens of thousands of vaccinated and unvaccinated people across the country.

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