Behind the masks, a mystery: how often do the vaccinated spread the virus?

New research showed that vaccinated people infected with the Delta variant carry tremendous amounts of the virus in the nose and throat, she said in an email responding to questions from The New York Times.

The finding contradicts what scientists had observed in vaccinated people infected with previous versions of the virus, who mostly seemed incapable of infecting others.

That conclusion dealt Americans a heavy blow: People with so-called breakthrough infections — cases that occur despite full vaccination — of the delta variant may be just as contagious as unvaccinated people, even if they have no symptoms.

That means fully immunized people with young children, aging parents, or friends and family with weak immune systems will need to renew vigilance, particularly in high-transmission communities. Vaccinated Americans may need to wear masks not just to protect themselves, but everyone in their orbit.

There are 67,000 new cases per day on average in the United States, as of Thursday. If vaccinated people are transmitting the delta variant, they may be contributing to the increases — although probably to a far lesser degree than the unvaccinated.

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

The research was conducted by people outside the CDC, the scientists said, and the agency is working quickly to analyze and publish the results. The agency expects to publish the research Friday, one official said.

Some of the research may be related in part to an outbreak in Provincetown, Massachusetts, where Fourth of July festivities have led to 882 cases as of Thursday. Nearly three-quarters of those people were fully vaccinated.

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

Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention (CDC), on Capitol Hill in Washington, July 20, 2021. (Stefani Reynolds/The New York Times)

It’s still unclear how common breakthrough infections are and how long the virus persists in the body in those cases. Breakthroughs are rare, and unvaccinated people account for the bulk of virus transmission, Walensky said.

Regardless, the data that the CDC is reviewing suggest that even fully immunized people can be unwilling vectors for the virus.

“We believe at individual level they might, which is why we updated our recommendation,” Walensky said in her email to The Times.

The conclusion also suggests that vaccinated people who are exposed to the virus should get tested, even if they feel fine. (In Britain, vaccinated people who are contacts of a known case are required to isolate for 10 days.)

The new data do not mean that the vaccines are ineffective. The vaccines still powerfully prevent severe illness and death, as they were meant to, and people with breakthrough infections very rarely end up in a hospital.

About 97% of people hospitalized with Covid-19 are unvaccinated, according to data from the CDC. But scientists warned even last year that the vaccines might not completely prevent infection or transmission. (Immunity from natural infection may offer even less protection.)

Previous versions of the virus rarely broke through the immunization barrier, which prompted 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 suggested that the amount of virus in unvaccinated people infected with delta might be a thousand times higher than seen in people infected with the original version of the virus. The CDC data support that finding, said one expert familiar with the results.

Anecdotes of clusters of breakthrough infections have become increasingly frequent, with groups of vaccinated people reporting sniffles, headache, sore throat, or a loss of taste or smell — symptoms of an infection in the upper respiratory tract. But the overwhelming majority do not end up needing intensive medical care, because the immune defenses produced by the vaccine destroy the virus before it can get to the lungs.