In yet another unexpected and unwelcome twist in the nation’s pandemic, fully immunized people with so-called breakthrough infections of the Delta variant may spread the virus to others just as easily as unvaccinated people, the Centers for Disease Control and Prevention said in a report published on Friday.
The vaccines remain powerfully effective against severe illness and death, and infections in vaccinated people are thought to be comparatively rare. But the revelation follows a series of other findings this week about the Delta variant, all of which have upended scientists’ understanding of the coronavirus.
An internal agency document, which was obtained on Thursday night by The New York Times, raised even more harrowing questions than the report released on Friday, which focused mainly on a huge cluster of infections in Provincetown, Mass.
Taken together, the data gathered by the C.D.C. throw into question the country’s plans to return to offices and schools this fall, and revive difficult questions about masking, testing and other precautions that Americans had hoped were behind them.
Most immediately, the research informed the agency’s decision this week to advise even vaccinated Americans to resume wearing masks in indoor public areas in communities where the virus is surging.
Even the vaccinated carry high virus levels if they become infected, the agency concluded, making it likely they can transmit the virus as often as the unvaccinated. If so, they may be contributing to increases in new infections — although probably to a far lesser degree than the unvaccinated.
“We spent so much time and energy and treasure trying to figure out this damn virus last year, and how it works and all the things it does,” said Dr. Robert Wachter, chairman of the Department of Medicine at the University of California, San Francisco.
Learning just how different the Delta variant is from the original virus is “just jarring,” he added. “The brain doesn’t like to keep being jerked around like this.”
Studies of outbreaks have shown that Delta is much more contagious than the original virus or the seasonal flu and as contagious as chickenpox, according to the internal document circulated within the C.D.C.
Breakthrough infections among vaccinated people were always anticipated, but until the Delta variant arrived, vaccinated Americans were not expected to be sources of new infections.
The vaccines remain the one reliable shield against the virus, in whatever form it takes. Nationwide, about 97 percent of people hospitalized with Covid-19 are unvaccinated, according to data from the C.D.C.
“Full vaccination is very protective, including against Delta,” said Angela Rasmussen, a research scientist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada.
“Masks are a wise precaution, but the bulk of transmission is among the unvaccinated and that’s still who is most at risk,” she added.
The gathering research underscores the urgency to pick up the pace of vaccination in the United States and decrease the numbers of people susceptible to severe illness. This week, the pace of vaccination in the European Union exceeded that in the United States for the first time.
The agency stepped up concern over the spread of the virus in its report on Friday, urging even jurisdictions with undetectable levels of the virus to put into effect precautions. But the C.D.C.’s internal document sounded a significantly more alarmed note, advocating for universal masking — for everyone, whatever the local transmission levels — and recommending that the agency “acknowledge the war has changed.”
With the number of daily cases up to nearly 72,000 cases on average as of Friday, immunized people with young children, aging parents, or friends and family with weak immune systems may need to wear masks to protect vulnerable people in their orbit — even in communities with lower infection rates.
Indeed, the questions now facing Americans seem nearly inexhaustible, almost insoluble. Should companies really return employees to workplaces if vaccinated people might on occasion spread the variant? What does this mean for shops, restaurants, schools? Are unmasked family gatherings again off the table?
Delta’s unpredictable nature has humbled scientists who had anticipated that the virus would cause mostly sporadic outbreaks in areas with low rates of vaccination. In Britain, where the variant seems to be subsiding after a surge, vaccinations were rolled out by age, and a much higher proportion of people over 50 are vaccinated than in the United States.
But vaccination rates are much more patchy in the United States, said Bill Hanage, an epidemiologist at the Harvard T. H. Chan School of Public Health. “The upshot is that what Delta does in the U.K. is not necessarily what it’s going to do in places which have more very varied vaccination,” he said.
“Things are going to be worse than they would have been” without the variant, he added. “But they’re going to be much better than they might have been without vaccination.”
In the report published on Friday, the agency described a single outbreak in Provincetown, Mass., that quickly mushroomed to nearly 469 cases in the state as of Thursday, three-quarters of whom were fully immunized.
The town’s experience this month strongly supports the idea that even fully immunized people can unwittingly spread the virus. “We believe at individual level they might, which is why we updated our recommendation,” Dr. Rochelle Walensky, director of the C.D.C., said in an email to The Times this week.
The outbreak sprouted after more than 60,000 revelers celebrated the Fourth of July in Provincetown, gathering in densely packed bars, restaurants, guesthouses and rental homes, often indoors.
By July 10, there was a noticeable uptick in cases among residents of the county, including among people who were fully immunized. A week later, the county’s daily average shot up from zero cases to 177 cases per 100,000 people.
“Vaccines are like hip waders,” Dr. Rasmussen said. “They keep you dry if you wade through a river, but get too deep and water will start pouring in over the top. That seems to be what happened in the Massachusetts outbreak.”
Three-quarters of the people infected in Provincetown reported having a cough, headache, sore throat or fever — symptoms of an infection in the upper airway — and 74 percent were known to be fully immunized.
Of the five people who were hospitalized, four were fully vaccinated — one with the Pfizer-BioNTech vaccine and three with the Johnson & Johnson vaccine. Two of the patients had underlying medical conditions. Genetic analysis of 133 cases identified the Delta variant in 119 and a closely related virus in one case.
Scientists warned even last year that the vaccines might not completely prevent infection or transmission. But experts did not expect that these events would figure significantly in the fight against the virus, nor did they anticipate how quickly the Delta variant would tear through the country.
“I thought two months ago that we were over the hump,” Dr. Wachter said. In San Francisco, the most highly vaccinated big city in the country, 77 percent of people over age 12 are vaccinated.
And yet, the hospital where he works has seen a sharp rise, from one case of Covid-19 on June 1, to 40 now. Fifteen of the patients are in intensive care.
“If getting to 70 or 75 percent immunity doesn’t protect the community, I think it’s very hard to extrapolate what happens to a place that is 30 percent vaccinated,” Dr. Wachter said. “Humility may be the most important thing here.”
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