New recommendations from federal health officials this week on when vaccinated Americans should don face masks came with a startling bolt of news: People who have had their shots and become infected with the delta variant of the coronavirus can harbor large amounts of virus just like unvaccinated people. That means they could become spreaders of the disease and should return to wearing masks indoors in certain situations, including when vulnerable people are present.

But the Centers for Disease Control and Prevention did not publish the new research. In the text of the updated masking guidance, the agency merely cited “CDC COVID-19 Response Team, unpublished data, 2021.”

Some outside scientists have their own message: Show us the data.

“They’re making a claim that people with delta who are vaccinated and unvaccinated have similar levels of viral load, but nobody knows what that means,” said Gregg Gonsalves, an associate professor at the Yale School of Public Health. “It’s meaningless unless we see the data.”

Top health officials Anthony S. Fauci and Rochelle Walensky said July 28 that vaccinated individuals should wear masks indoors, per new CDC guidance. (The Washington Post)

When CDC Director Rochelle Walensky spoke to reporters Tuesday, she cited the “new scientific data” but provided limited details about how the research was done. She said the data comes from outbreak investigations in which researchers compared delta infections among vaccinated and unvaccinated people.

The data will be “published imminently,” according to a federal official knowledgeable about the research but who was not authorized to be a spokesperson for the government.

“These data were alarming and recently presented,” the official said Wednesday. “We saw the data and thought it was urgent enough to act — in the context of a steeply rising, preventable fourth surge of covid-19.”

Because tests showed similar levels of virus in the vaccinated and unvaccinated, the CDC inferred the delta variant can be transmitted by people with breakthrough infections.

“I think the implications [of the data] are that people who are vaccinated, even when they’re asymptomatic, can transmit the virus, which is the scientific foundation of why this recommendation is being made,” Anthony S. Fauci, President Biden’s chief medical adviser, said in an interview with The Washington Post.

But Fauci noted there is not yet clinical data on what the high viral loads mean in terms of disease transmission. “You can make a reasonable assumption that vaccinated people can transmit the virus just like unvaccinated people can,” Fauci said.

Three senior administration officials who spoke on the condition of anonymity to describe internal discussions said the new research convinced health officials that it was time to update the agency’s guidance. When scientists compared viral loads in vaccinated and unvaccinated individuals infected with an earlier variant of the virus — the alpha variant, which was dominant in the spring — there were considerable differences in the amount of virus each carried.

The CDC did not answer questions Wednesday about whether it relied on outside sources of data or the number of patients examined in its outbreak investigations.

The medical and scientific community has generally endorsed the change in CDC mask guidance. Several organizations and public health experts issued statements saying the CDC should have gone further and broadened the criteria for deciding which communities have transmission high enough to warrant universal masking indoors.

The question about viral loads is among the many unknowns surrounding SARS-CoV-2, including the frequency of breakthrough infections and whether they play a significant role in the recent rise in cases.

“If we’re seeing more breakthroughs, is it just because the virus is better and the vaccines don’t hold up quite as well, or is the efficacy of the vaccines beginning to wane, independent of the delta?” asked Robert Wachter, chair of the Department of Medicine at the University of California at San Francisco. “This is three-dimensional chess, there’s a hundred things going on at the same time.”

There is now a Greek-alphabet soup of viral variants competing with one another. The delta, which was first identified in the United States in February and only gained traction in June, is dominant in the United States.

“The big concern is that the next variant that might emerge, just a few mutations away, could potentially evade our vaccine,” Walensky said Tuesday.

There are multiple vaccines deployed to stop the pandemic, with a range of efficacy in stopping mild infections. The vaccines are all highly protective against severe disease and death. Pfizer published data Wednesday showing a modest drop in efficacy over the course of six months.

Although delta is more than twice as transmissible as earlier variants, it does not have some of the mutations seen in other variants that can help the virus evade antibodies. But the delta floods the zone. It grows so quickly in the nose that it may be overwhelming the body’s vaccine-enhanced defenses before the immune system can marshal a robust response, said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

“The immune response, once activated, takes a while to kick in even among people who have been vaccinated,” Hanage said in an email. “As a result if the virus can copy itself really quickly it might be able to get a few rounds of replication in, even in vaccinated folks, before the immune system brings it under control.”

The Singapore Ministry of Health recently found that three-fourths of coronavirus cases in the past four weeks were in people who were fully or partially vaccinated, most with no or mild symptoms. And in India, vaccinated health-care workers showed high viral loads when infected with the delta, according to a study from University of Cambridge researchers that is not yet peer-reviewed.

The senior author of that study, microbiologist Ravindra Gupta, said the infectivity of people with breakthrough infections has not been “formally measured in a rigorous way,” but the new research shows high viral loads in people with breakthrough delta infections. That suggests vaccinated people should wear masks, he said.

Research by Chinese scientists posted online and not yet peer-reviewed describes the stunning ability of the delta variant to replicate in the human body. The viral load from the delta is 1,000 times that detected in the earliest variants of the virus. That is about 10 times the viral load sparked by the alpha variant, which was first seen in the United Kingdom and became dominant in the United States this spring before the delta overcompeted it.

“Delta is alpha on steroids,” said James M. Musser, chair of the Department of Pathology and Genomic Medicine at Houston Methodist Hospital and Research Institute.

In the eight hospitals run by Houston Methodist, there are about 300 covid-19 patients, triple the number in early June, Musser said. Most new cases involve the delta variant. He estimated that 20 percent of the covid patients were fully vaccinated before becoming infected.

But he cautioned that most of these patients have underlying medical conditions that impaired their ability to mount an immune response after being vaccinated.

These post-vaccination infections have often been described by Walensky and other medical experts as rare. How rare is unclear. News reports of people getting sick after vaccination have been common in recent weeks. But scientific data is limited.

The CDC on May 1 said it would stop tracking mild and moderate breakthrough cases, and focus only on hospitalizations and deaths. As of July 19, the CDC had documented 5,914 such breakthroughs, including 1,141 deaths.

CDC spokeswoman Kristen Nordlund said Wednesday the agency conducts “cohort” studies to obtain estimates of the efficacy of the vaccines that often involve tens of thousands of people. Scientists examine vaccinated and unvaccinated patients for a period of time to see if they develop covid-19, Nordlund said.

Several experts have criticized the agency for not tracking mild and moderate breakthrough cases on a broader scale, arguing it makes it difficult to know how rare these cases really are.

Even though the vaccines remain effective against all variants of the coronavirus, they are not designed to create “sterilizing” immunity. That’s why breakthrough infections happen. The virus can infect the nose and begin replicating before the immune system rallies its range of defenses. The vaccines prime the immune system, including the “B memory” cells that begin cranking out antibodies after detection of an invasive pathogen.

Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia, said it’s like the fire extinguisher in your kitchen. The immune system ensures you have that fire extinguisher standing by for an emergency. But it can’t prevent the initial conflagration. “You still had a little fire in the kitchen,” Offit said.

Larry Corey, a virologist at the Fred Hutchinson Cancer Research Center, said it would not be surprising to see a variant emerge that is better at replicating in people’s noses. Animal studies, he said, indicated that vaccines were better at protecting animals’ lungs from infection than their noses. That might help explain why vaccinated people can become infected but rarely develop severe disease.

“The virus is under selective pressure to jump from nose to nose,” Corey said. “So its evolutionary sort of pressure is to do that as efficiently as it can. Delta is more efficient than others.”

Even if tests find lots of virus in vaccinated people, it is uncertain how contagious they are. A study of immunized health-care workers in Israel, published Wednesday in the New England Journal of Medicine, found 39 breakthrough infections among 1,497 fully vaccinated people. About three-fourths of those people had, at some point while infected, what researchers characterized as high viral loads. There was no evidence that a breakthrough case led to other infections.

Natalie Dean, a biostatistics expert at Emory University’s Rollins School of Public Health, said she remains unconvinced a high viral load in the nose truly means that vaccinated and unvaccinated people are equally as likely to spread the virus, although she acknowledged there is an ongoing debate about the issue.

“I feel like nasal viral load is one part of a lot of other parts” that determine how infectious a person is, Dean said, adding that she thinks the amount of virus in the throat or lungs could be important and might differ between people who are vaccinated and those who are not.