The recommendation that vaccinated people dust their masks off in some parts of the country was based, according to Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, is largely based on a problematic finding.

New research showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in their noses and throats, she said in an email in response to questions from the New York Times.

The finding contradicts what scientists had observed in vaccinated people who were infected with previous versions of the virus and who mostly appeared unable to infect others.

This conclusion dealt a severe blow to Americans: people with what are known as breakthrough infections – cases that occur despite being fully vaccinated – of the Delta variant can be just as contagious as unvaccinated people, even if they have no symptoms.

This means that fully vaccinated people with young children, aging parents, or friends and family with weak immune systems need to renew their vigilance, especially in communities with high transmission. Vaccinated Americans may need to wear masks not only to protect themselves but everyone around them.

In the US, there are an average of 67,000 new cases per day as of Thursday. If vaccinated people transmit the Delta variant, they can contribute to the increases – albeit likely to a far lesser extent than those who were not vaccinated.

The CDC has not yet released its data, frustrating experts who want to understand the basis for the change of heart with masks. Four scientists familiar with the research said it was imperative and based on the advice of the CDC that those vaccinated again wear masks in public indoor spaces.

The research was carried out by people outside the CDC, the scientists said, and the agency is working quickly to analyze and publish the results. The agency expects to release the research on Friday, an official said.

Some of the research may be related in part to an outbreak in Provincetown, Massachusetts, where the July 4th celebrations through Thursday resulted in 882 cases. Almost three quarters of these people were fully vaccinated.

The agency has also tracked data from the Covid-19 Sports and Society Workgroup, a coalition of professional sports leagues that tests more than 10,000 people at least every day and sequences all infections.

It is still unclear how common breakthrough infections are and how long the virus remains in the body in these cases. Breakthroughs are rare and unvaccinated people are responsible for the majority of virus transmission, said Dr. Walensky.

Regardless, the data the CDC is reviewing suggests that even fully immunized people can be unwilling vectors for the virus. “We believe this can be done on an individual level, which is why we have updated our recommendation,” said Dr. Walensky in her email to the Times.

The conclusion also suggests that vaccinated people exposed to the virus should get tested even if they feel fine. (In the UK, vaccinated individuals who come into contact with a known case are required to isolate themselves for 10 days.)

Updated

July 30, 2021, 10:41 p.m. ET

The new data doesn’t mean the vaccines are ineffective. The vaccines are still effective in preventing serious illness and death as they were intended, and people with breakthrough infections very rarely end up in hospital.

According to data from the CDC, around 97 percent of people hospitalized with Covid-19 are unvaccinated. (Immunity to a natural infection can offer even less protection.)

Earlier versions of the virus rarely broke the immunization barrier, prompting the CDC to advise in May that vaccinated people could go mask-free indoors. But the usual rules don’t seem to apply to the delta variant.

The variant is twice as contagious as the original virus, and one study found that the amount of virus in unvaccinated people infected with Delta could be a thousand times higher than in people infected with the original version of the virus. The CDC data support this finding, said an expert familiar with the results.

Anecdotes of clusters of breakthrough infections are becoming more common, with groups of those vaccinated reporting runny nose, headache, sore throat, or loss of taste or smell – symptoms of an upper respiratory infection.

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The overwhelming majority, however, do not need intensive care because the immune system created by the vaccine destroys the virus before it can reach the lungs.

“We’re still going to see a huge, huge impact on disease severity and hospitalization,” said Michal Tal, an immunologist at Stanford University. “That’s what the vaccine was really made for.”

The coronavirus vaccines are injected into the muscle, and most of the antibodies produced in response remain in the blood. Some antibodies can get into the nose, the main entry point for the virus, but not enough to block it.

“The vaccines – they’re beautiful, they work, they’re amazing,” said Frances Lund, a viral immunologist at the University of Alabama in Birmingham. “But they won’t give you that local immunity.”

When people are exposed to a respiratory pathogen, it can gain a foothold in the lining of the nose – without causing any further damage. “If you walked down the street and wiped people, you’d find people with viruses in their lining that were asymptomatic,” said Dr. Michael Marks, epidemiologist at the London School of Hygiene and Tropical Medicine. “Our immune systems fight these things most of the time.”

But the Delta variant seems to thrive in the nose, and its abundance could explain why more people than scientists expected are experiencing breakthrough infections and cold-like symptoms.

However, when the virus tries to snake its way into the lungs, the immune cells in vaccinated people are fired up and quickly clear the infection before it does any major damage. That means vaccinated people should be infected and contagious for a much shorter period of time than unvaccinated people, said Dr. Lund.

“But that doesn’t mean they can’t pass it on to someone else for the first few days when they’re infected,” she added.

To stop the virus right where it enters, some experts have recommended nasal spray vaccines that would prevent the intruder from entering the upper respiratory tract. “Vaccine 1.0 is designed to prevent death and hospitalization. Vaccine 2.0 should prevent transmission, ”said Dr. Valley. “We just need one more iteration.”