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A dose of the Pfizer-BioNTech vaccine being administered at a three-day clinic in Wilmington, Calif., on Thursday. Credit…Mario Tama/Getty Images

Apoorva Mandavilli

By Apoorva Mandavilli – Published July 30, 2021 – Updated July 31, 2021, 10:28 a.m. ET

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.

Coronavirus Pandemic and U.S. Life Expectancy

Government officials and scientists alike are gravely concerned that the findings may shake faith in the vaccines, hobbling the nation’s 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.”

Is this helpful?

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.