In another unexpected and unwelcome twist in the pandemic, the Centers for Disease Control and Prevention released a report on Friday strongly suggesting that fully immunized people with so-called breakthrough Delta variant infections can pass the virus on to others as easily as unvaccinated people People.

The vaccines remain highly effective against serious illness and death, and the agency said infections are comparatively rare in people who have been vaccinated. But the reveal follows a number of other recent discoveries about the Delta variant that have turned scientists’ understanding of the coronavirus on its head.

In the new report, which should explain the agency’s sudden revision of its masking recommendations for vaccinated Americans, the CDC described an outbreak in Provincetown, Massachusetts this month that rose rapidly to 470 cases in Massachusetts alone by Thursday.

Three quarters of those infected were fully immunized, and the Delta variant was found in most of the genetically analyzed samples. Vaccinated and unvaccinated people who were infected carried high levels of the virus, the agency reported.

“High viral loads indicate an increased risk of transmission and raised concerns that, unlike other variants, people infected with Delta can transmit the virus,” said Dr. Rochelle Walensky, director of the CDC, on Friday.

The viral load data shows that even fully vaccinated people can spread the virus just as easily as unvaccinated people who become infected. “We believe this can be done on an individual level, which is why we have updated our recommendation,” said Dr. Walensky in an email to the New York Times earlier this week.

An internal agency document the Times received Thursday evening indicated even greater concern among CDC scientists, raising harrowing questions about the virus and its trajectory.

The delta variant is about as contagious as chickenpox, the document says, and universal masking may be necessary. Nevertheless, according to the agency, breakthrough infections are rare overall.

On Friday, the Kaiser Family Foundation reported that the breakthrough rate among fully vaccinated people in states that store such data is less than 1 percent.

The accumulated research on the variant messes up the country’s plans to return to offices and schools this fall, and enlivens tough questions about masking, testing, and other precautions that Americans had hoped were behind them.

Government officials and scientists alike are seriously concerned that the results could shake confidence in the vaccines and shake the nation’s delayed vaccination campaign if Americans mistakenly conclude that the vaccinations are not effective.

Concerned about the delayed campaign, President Biden has ordered all federal employees to be vaccinated or tested for viruses on a weekly basis. Support for vaccination regulations is growing at some companies and in some parts of the country.

Developing research into the Delta variant has humiliated scientists around the world who are now asking themselves new questions about the virus that they had not considered.

They do not understand the circumstances that can increase the likelihood of a breakthrough infection, nor who is most at risk. They don’t know for sure that the Delta variant causes more severe illness in unvaccinated people who become infected, although early data suggests it.

“We spent so much time, energy, and treasure last year trying to figure out this damn virus and how it works and what it does,” said Dr. Robert Wachter, chairman of the Department of Medicine at the University of California. San Francisco.

To learn how differently the Delta variant differs from the original virus is “just plain staggering,” he added. “The brain doesn’t like being pushed around like that.”

While breakthrough infections are rare, the new data suggest that those who were vaccinated may contribute to an increase in new infections – albeit likely to a far lesser extent than those who were not. Breakthrough infections have always been reckoned with, but until the arrival of the Delta variant, vaccinated Americans were not seen as drivers of its spread in the community.

“Delta teaches us to expect the unexpected,” said John Moore, a virologist at Weill Cornell Medicine in New York. “There are aspects of what we now know that we don’t see coming.”


July 30, 2021, 7:36 p.m. ET

The finding is frightening, but vaccines remain the only reliable shield against the virus in whatever form. Even with the Delta variant, the vaccines largely prevent infection and significantly reduce the likelihood of serious illness or death in the event of an infection.

Nationwide, about 97 percent of people hospitalized with Covid-19 are unvaccinated, according to the CDC.

“Full vaccination is very protective, even against Delta,” said Angela Rasmussen, researcher at the Organization for Vaccines and Infectious Diseases at the University of Saskatchewan in Canada.

“Masks are a wise precaution, but most of the transmission occurs among the unvaccinated and that is still the most at risk,” she added.

The accumulated research underscores the urgency to accelerate the rate of vaccination in the United States and reduce the number of people susceptible to serious illnesses. This week, the vaccination rate in the European Union exceeded that in the US for the first time.

About 58 percent of Americans 12 years and older are fully vaccinated. The rate of vaccination has slowed to just over 500,000 people a day, although it has swung up slightly in recent weeks as infections pick up again.

In the UK, where the variant seems to have subsided after an increase, vaccinations have been introduced by age and a much higher proportion of people over 50 are vaccinated than in the United States.

Understand the state of vaccine mandates in the United States

Vaccination rates are much more inconsistent across the United States, said Bill Hanage, an epidemiologist at Harvard TH Chan School of Public Health. “The result is that what Delta is doing in the UK is not necessarily what it will be doing in places with very different vaccinations,” he said.

“Things are getting worse than they would have been,” without the variant, he added. “But they will be much better than they would have been without the vaccination.”

In its report on Friday, the CDC urged local and state officials in jurisdictions with even lower virus concentrations to consider precautions such as masking and restricting gatherings. The CDC internal document sounded more urgent, recommending that the agency “recognize that the war has changed”.

Indeed, the questions Americans now face seem almost inexhaustible, almost insoluble. Should companies allow employees to return to work when vaccinated people could occasionally spread the variant? What does this mean for shops, restaurants and schools? Are unmasked family celebrations off the table again?

With the number of daily cases averaging nearly 72,000 on Friday, the new data suggests vaccinated people with young children, aging parents, or friends and family with weak immune systems may need to wear masks to protect those around them – even in Communities with lower infection rates.

The Provincetown, Massachusetts outbreak germinated this month after more than 60,000 revelers celebrated the July 4th gathering in crowded bars, restaurants, guest houses and rental apartments, often indoors.

On July 3, there were no cases in the city or the surrounding district. By July 10, officials saw an increase 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 waders,” said Dr. Rasmussen. “They keep you dry when you wade through a river, but when you get too deep, water starts flowing over it. That seems to have happened with the Massachusetts eruption. “

Three-quarters of citizens linked to the outbreak reported a cough, headache, sore throat, or fever – symptoms of an upper respiratory tract infection – and 74 percent were known to be fully immunized.

Of the five people hospitalized, four were fully vaccinated – one with the Pfizer BioNTech vaccine and three with the Johnson & Johnson vaccine. Two of the vaccinated patients had previous illnesses. The genetic analysis of 133 cases identified the delta variant in 119 cases and a closely related virus in another case.

Scientists even warned last year that the vaccines may not completely prevent infection or transmission. However, experts didn’t expect these infections to play a significant role in the fight against the virus, nor did they anticipate how quickly the Delta variant would rip across the country.

“Two months ago I thought we were over the top,” said Dr. Guardian. In San Francisco, the most heavily vaccinated city in the country, 77 percent of people over the age of 12 are vaccinated.

And yet the hospital he works in has grown significantly, from a Covid-19 case on June 1 to 40 now. 15 of the patients are in intensive care.

“When a 70 or 75 percent immunity doesn’t protect the community, I think it’s very difficult to extrapolate what happens to a place that is 30 percent vaccinated,” said Dr. Guardian. “Humility is perhaps the most important thing here.”