In yet another unexpected and unwelcome twist in the pandemic, the Centers for Disease Control and Prevention released on Friday a report strongly suggesting that fully immunized people with so-called breakthrough infections of the Delta variant can spread the virus to others just as readily as unvaccinated people.
The vaccines remain powerfully effective against severe illness and death, and the agency said infections in vaccinated people were comparatively rare. But the revelation follows a series of other recent findings about the Delta variant that have upended scientists’ understanding of the coronavirus.
In the new report, which was intended to explain the agency’s sudden revision to its masking advice for vaccinated Americans, the C.D.C. described an outbreak in Provincetown, Mass., this month that quickly mushroomed to 470 cases in Massachusetts alone, as of Thursday.
Three-quarters of the infected were fully immunized, and the Delta variant was found in most of the samples that were genetically analyzed. Vaccinated and unvaccinated people who were infected carried high levels of the virus, the agency reported.
“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, director of the C.D.C., said on Friday.
The viral load data indicate that even fully immunized people can spread the virus as easily as unvaccinated people who become infected. “We believe at individual level they might, which is why we updated our recommendation,” Dr. Walensky said in an email to The New York Times earlier this week.
An internal agency document, which was obtained on Thursday night by The Times, suggested even greater alarm among C.D.C. scientists and raised harrowing questions about the virus and its trajectory.
The Delta variant is about as contagious as chickenpox, the document noted, and universal masking may become necessary. Still, breakthrough infections overall are infrequent, according to the agency.
On Friday, the Kaiser Family Foundation reported that the rate of breakthrough cases is less than 1 percent among fully vaccinated people in states that keep such data.
The gathering research into the variant throws into disarray the country’s plans to return to offices and schools this fall, and revives difficult questions about masking, testing and other precautions that Americans had hoped were behind them.
Government officials and scientists alike are gravely concerned that the findings may shake faith in the vaccines, hobbling the nation’s lagging immunization campaign, should Americans infer incorrectly that the shots are not effective.
Concerned by the lagging campaign, President Biden has ordered that all federal employees be vaccinated or face weekly virus testing. Support for vaccination mandates is growing among some corporations and in some parts of the country.
The evolving research into the Delta variant has humbled scientists worldwide, who now confront fresh questions about the virus they had not considered.
They do not fully understand the circumstances that may increase the odds of a breakthrough infection, for example, nor who may be most at risk. They do not know for certain that the Delta variant causes more severe disease in the unvaccinated who become infected, although early data suggest it does.
“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.”
Even if breakthrough infections are rare, the new data suggest the vaccinated may be contributing to increases in new infections — although probably to a far lesser degree than the unvaccinated. Breakthrough infections were always anticipated, but until the Delta variant arrived, vaccinated Americans were not believed to be drivers of community spread.
“Delta is teaching us to expect the unexpected,” said John Moore, a virologist at Weill Cornell Medicine of New York. “There are aspects of what we now know that we didn’t see coming.”
The finding is dismaying, but vaccines remain the one reliable shield against the virus, in whatever form it takes. The vaccines largely prevent infection, even with the Delta variant, and greatly reduce the chances of severe illness or death should infection occur.
Nationwide, about 97 percent of people hospitalized with Covid-19 are unvaccinated, according to data from the C.D.C. And the unvaccinated are far more likely to spread the virus to others in their communities.
“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 rate of vaccination in the European Union exceeded that in the United States for the first time.
About 58 percent of Americans ages 12 and older are fully vaccinated. The pace of vaccination has slowed to just over 500,000 people per day, although it has begun curving slightly upward in the past couple of weeks as infections rise again.
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.
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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 its report on Friday, the C.D.C. urged local and state officials in jurisdictions with even lower levels of the virus to considering putting into effect precautions, such as masking and limiting gatherings. The C.D.C.’s internal document sounded more urgent, recommending that the agency “acknowledge the war has changed.”
Indeed, the questions now facing Americans seem nearly inexhaustible, almost insoluble. Should companies have employees return to workplaces if vaccinated people might, on occasion, spread the variant? What does this mean for shops, restaurants and schools? Are unmasked family gatherings again off the table?
With the number of daily cases up to nearly 72,000 on average as of Friday, the new data suggest that 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.
The outbreak in Provincetown, Mass., this month sprouted after more than 60,000 revelers celebrated the Fourth of July gathering in densely packed bars, restaurants, guesthouses and rental homes, often indoors.
On July 3, there were no cases in the town and surrounding county. By July 10, officials noted an uptick, and by July 17, there were 177 cases per 100,000 people. The outbreak has since spread to nearly 900 people across the country.
“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 state residents linked to the outbreak 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 vaccinated patients had underlying medical conditions. Genetic analysis of 133 cases identified the Delta variant in 119 and a closely related virus in one additional case.
Scientists warned even last year that the vaccines might not completely prevent infection or transmission. But experts did not expect that these infections 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.”