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 In the text of the updated masking guidance, the agency merely cited “CDC COVID-19 Response Team, unpublished data, 2021.”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.”
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.
Experts ask to see data behind new policy
Joel Achenbach, et al
"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 Fauci, President Joe 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.
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
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.
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