In another dispiriting setback for the nation’s efforts to stamp out the coronavirus, scientists who studied a big COVID-19 outbreak in Massachusetts concluded that vaccinated people who got so-called breakthrough infections carried about the same amount of the coronavirus as those who did not get the shots. Health officials on Friday released details of that research, which was key in this week’s decision by the Centers for Disease Control and Prevention to recommend that vaccinated people return to wearing masks indoors in parts of the U.S. where the delta variant is fueling infection surges. The authors said the findings suggest that the CDC’s mask guidance should be expanded to include the entire country, even outside of hot spots. The findings have the potential to upend past thinking about how the disease is spread. Previously, vaccinated people who got infected were thought to have low levels of virus and to be unlikely to pass it to others. But the new data shows that is not the case with the delta variant. The outbreak in Provincetown — a seaside tourist spot on Cape Cod in the county with Massachusetts’ highest vaccination rate — has so far included more than 900 cases. About three-quarters of them were people who were fully vaccinated. Travis Dagenais, who was among the many vaccinated people infected, said “throwing caution to the wind” and partying in crowds for long nights over the July Fourth holiday was a mistake in hindsight. “The dominant public messaging has been that the vaccine means a return to normal,” the 35-year-old Boston resident said Thursday. “Unfortunately, I’ve now learned it’s a few steps toward normal, not the zero-to-sixty that we seem to have undertaken.” Dagenais credits being vaccinated with easing the worst of the flu-like symptoms in a couple of days. He has recovered. Like many states, Massachusetts lifted all COVID-19 restrictions in late May, ahead of the traditional Memorial Day start of the summer season. Provincetown this week reinstated an indoor mask requirement for everyone. Leaked internal documents on breakthrough infections and the delta variant suggest the CDC may be considering other changes in advice on how the nation fights the coronavirus, such as recommending masks for everyone and requiring vaccines for doctors and other health workers. Read: Do I need to get tested for COVID-19 if I’m vaccinated? The delta variant, first detected in India, causes infections that are more contagious than the common cold, flu, smallpox and the Ebola virus, and it is as infectious as chickenpox, according to the documents, which mentioned the Provincetown cases. The documents were obtained by The Washington Post. As they note, COVID-19 vaccines are still highly effective against the delta variant at preventing serious illness and death. The Provincetown outbreak and the documents highlight the enormous challenge the CDC faces in encouraging vaccination while acknowledging that breakthrough cases can occur and can be contagious but are uncommon. The documents appear to be talking points for CDC staff to use with the public. One point advised: “Acknowledge the war has changed,” an apparent reference to deepening concern that many millions of vaccinated people could be a source of wide-ranging spread. An agency spokeswoman declined to comment on the documents. The White House on Friday defended its approach to rising virus cases and shifting public health guidelines, repeatedly deferred to the CDC while stressing the need for vaccinations. “The most important takeaway is actually pretty simple. We need more people to get vaccinated,” White House spokeswoman Karine Jean-Pierre said. Pressed about the changing guidance, Jean-Pierre repeatedly said, “We don’t make those types of decisions from here.” People with breakthrough infections make up an increasing portion of hospitalizations and in-hospital deaths among COVID-19 patients, coinciding with the spread of the delta variant, according to the leaked documents. Although experts generally agreed with the CDC’s revised indoor masking stance, some said the report on the Provincetown outbreak does not prove that vaccinated people are a significant source of new infections. Read: Should vaccinated people mask up with COVID-19 cases rising? “There’s scientific plausibility for the (CDC) recommendation. But it’s not derived from this study,” said Jennifer Nuzzo, a Johns Hopkins University public health researcher. The CDC report is based on about 470 COVID-19 cases linked to the Provincetown festivities, which included densely packed indoor and outdoor holiday events at bars, restaurants, guest houses and rental homes. Researchers ran tests on a portion of them and found roughly the same level of virus in those who were fully vaccinated and those who were not. Three-quarters of the infections were in fully vaccinated individuals. Among those fully vaccinated, about 80% experienced symptoms with the most common being cough, headache, sore throat, muscle aches and fever. Dagenais said he started to feel ill the evening he returned home and initially chalked it up to long nights of partying in packed Provincetown nightclubs. But as the days wore on and the fever, chills, muscle aches and fatigue set in, he knew it was something more. In the report, the measure researchers used to assess how much virus an infected person is carrying does not indicate whether they are actually transmitting the virus to other people, said Dr. Angela Rasmussen, a virologist at the University of Saskatchewan. CDC officials say more data is coming. They are tracking breakthrough cases as part of much larger studies that involve following tens of thousands of vaccinated and unvaccinated people across the country over time.

Add travel to the activities vaccinated Americans can safely enjoy again, according to new U.S. guidance issued Friday. The Centers for Disease Control and Prevention updated its guidance to say fully vaccinated people can travel within the U.S. without getting tested for the coronavirus or going into quarantine afterward. Still, CDC Director Dr. Rochelle Walensky urged caution and said she would “advocate against general travel overall" given the rising number of infections. “If you are vaccinated, it is lower risk,” she said. According to the CDC, nearly 100 million people in the U.S. — or about 30% of the population — have received at least one dose of a COVID-19 vaccine. A person is considered fully vaccinated two weeks after receiving the last required dose. Also read: Fully-vaccinated people can gather without masks: CDC The agency had said it would update its guidance as more people got vaccinated and evidence mounted about the protection the shots provide. “Every day you get more data, and you change your guidance," said Dr. Ali Khan, dean of the University of Nebraska's College of Public Health. The CDC is sticking to its guidance for unvaccinated people to avoid unnecessary travel. If they do travel, the agency says to get tested one to three days before the trip, and three to five days after. People should also stay home and quarantine for seven days after travel, even if their COVID-19 test is negative, the agency says. The new guidance says: — Fully vaccinated people can travel within the U.S., without getting tested for the coronavirus or quarantining. People should still wear a mask, socially distance and avoid crowds, the agency says. — For international travel, the agency says vaccinated people do not need to get a COVID-19 test before leaving, unless the destination country requires it. — For travelers coming into the U.S., vaccinated people should still get a negative COVID-19 test before boarding a flight, and be tested three to five days after arrival. They do not need to quarantine. The agency noted the potential introduction of virus variants and differences in vaccine coverage around the world for the cautious guidance on overseas travel. Also read: CDC: Strong evidence in-person schooling can be done safely Already, air travel in the United States has been picking back up. Although traffic remains down by nearly half from a year ago, more than 1 million travelers daily have been going through U.S. airports in recent weeks. Airlines do not require COVID-19 tests or proof of vaccination for travel in the U.S. The CDC cited recent research on the real-world effects of the vaccines for its updated guidance. Last month, the agency said fully vaccinated people could visit with each other indoors without wearing masks or social distancing. It also said vaccinated people could visit with unvaccinated people from a single household under similar conditions, as long as the unvaccinated individuals were at low risk for severe illness if infected. The U.S. began its vaccine rollout in mid-December. Vaccines from Pfizer and Moderna require two doses given a few weeks apart. A one-shot vaccine by Johnson & Johnson was given the green light by regulators at the end of February.

Fully-vaccinated Americans can gather with other vaccinated people indoors without wearing a mask or social distancing, according to long-awaited guidance from federal health officials.