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

According to Dr Rochelle Walensky, director of the Centers for Disease Control and Prevention, the recommendation that people vaccinated in parts of the country dust off their masks was largely based on an embarrassing finding.

New research has shown that vaccinated people infected with the Delta variant carry huge amounts of the virus in their 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, most of whom seemed unable to infect others.

This finding was a blow to Americans: People with so-called breakthrough infections – cases that occur despite a full vaccination – of the Delta variant can be just as contagious as unvaccinated people, even if they don’t have any. symptom.

This means that fully immunized people with young children, aging parents, or friends and family with weakened immune systems will need to be vigilant, especially in high transmission communities. Vaccinated Americans may need to wear masks not just for protection, but for anyone in their orbit.

There are 67,000 new cases per day on average in the United States on Thursday. If vaccinated people transmit the Delta variant, they may contribute to the increases – although likely to a much lesser degree than the unvaccinated.

The CDC has yet to release its data, frustrating experts who want to understand the basis for the change in mind on masks. Four scientists familiar with the research said it was compelling and justified the CDC’s opinion that vaccinees were once again wearing masks in indoor public 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 plans to publish the research on Friday, an official said.

Some of the research may be linked in part to an outbreak in Provincetown, Massachusetts, where the July 4 festivities led to 882 cases on Thursday. Almost three quarters of these people were fully vaccinated.

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

It is still not clear how common breakthrough infections are and how long the virus persists in the body in these cases. Breakthroughs are rare and unvaccinated people account for most of the transmission of the virus, Dr Walensky said.

Regardless, the data the CDC is examining suggests that even fully immune people may be reluctant vectors for the virus. “We believe that on an individual level they could do this, which is why we have updated our recommendation,” Dr Walensky said in his email to The Times.

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

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

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

Previous versions of the virus rarely crossed the vaccination barrier, prompting the CDC to report in May that those vaccinated could go indoors without a mask. 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 may be a thousand times higher than in people infected with the original version of the virus. CDC data supports this conclusion, said an expert familiar with the findings.

Anecdotes of clusters of breakthrough infections have become increasingly common, with groups of vaccinated people reporting sniffles, headaches, sore throats, or loss of taste or smell – symptoms of an infection. upper respiratory tract.

But the overwhelming majority do not need intensive medical attention, as the immune defenses produced by the vaccine destroy the virus before it can reach the lungs.

“We’re always going to see a huge, huge, huge impact on the severity of the disease and the hospitalization,” said Michal Tal, an immunologist at Stanford University. “This is really what the vaccine was designed to do.”

Vaccines against the coronavirus are injected into the muscle, and the antibodies produced in response mostly remain in the blood. Some antibodies can make their way to the nose, the main gateway for the virus, but not enough to block it.

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

When people are exposed to a respiratory pathogen, it can implant itself in the nasal mucosa, without causing damage beyond that. “If you walked down the street and scouted people, you would find people who had viruses in their lining that were asymptomatic,” said Dr Michael Marks, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Our immune system primarily fights these things most of the time.”

But the Delta variant appears to thrive in the nose, and its abundance may explain why more people than scientists predict are suffering from breakthrough infections and cold-like symptoms.

Yet, when the virus tries to make its way into the lungs, the immune cells of those vaccinated will intensify and quickly clear the infection before it does much damage. This means that vaccinated people are expected to be infected and contagious for a much shorter period of time than unvaccinated people, Dr Lund said.

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

To stop the virus where it enters, some experts have advocated nasal spray vaccines that would prevent the invader from entering the upper respiratory tract. “The 1.0 vaccine is expected to prevent death and hospitalization. The 2.0 vaccine is expected to prevent transmission, ”said Dr Tal. “We just need another iteration.”