CDC on Tuesday released new mask guidance saying everyone—regardless of vaccination status—should wear masks indoors in public in areas with surging Covid-19 case rates, updating its guidance from two months ago that said fully vaccinated people can forgo masks in most situations.

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What's in CDC's new guidance

CDC in the latest guidance recommended that everyone—including fully vaccinated people—wear masks in public indoor spaces in counties that CDC has categorized as having "high" or "substantial" Covid-19 transmission.

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CDC categorizes each U.S. county as "low," "moderate," "substantial," or "high" transmission based on a combination of disease prevalence and the percentage of positive Covid-19 tests. Details of the categorization criteria, and a tool to look up each county's risk level, are available on CDC's website.

According to CNN, 46% of U.S. counties currently have "high" Covid-19 transmission, while 17% have "substantial" transmission.

CDC in the guidance reiterated its previous recommendation that schools return for in-person learning this fall, but the agency now recommends universal masking for students, teachers, visitors, and staff, regardless of vaccination status.

Masking is a 'temporary measure,' Walensky says

According to CDC Director Rochelle Walensky, the update was prompted by new data showing that while unvaccinated people still account for the vast majority of coronavirus transmission, vaccinated people with breakthrough cases of Covid-19 "may be contagious and spread the virus to others."

Specifically, she said CDC officials have found that the levels of the virus in vaccinated people who have contracted Covid-19 is "similar" to the amount of the virus in infected people who have not been vaccinated. Walensky said this may be a characteristic of infections caused by the delta variant, as earlier research on the alpha variant suggested that fully vaccinated people were not transmitting the virus.

"In recent days I have seen new scientific data from recent outbreak investigations showing that that delta variant behaves uniquely differently from past strains of the virus that cause Covid-19," she said. "This new science is worrisome and unfortunately warrants an update to our recommendations."

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Nonetheless, Walensky added that the currently available vaccines are powerful tools against the coronavirus, stating that they reduce the risk of symptomatic infection seven-fold and the risk of hospitalization 20-fold. She recommended that community leaders continue to encourage vaccination and mask wearing to prevent further outbreaks.

Walensky added that masking is only a "temporary measure." She said, "What we really need to do to drive down these transmissions in the areas of high transmission is to get more and more people vaccinated and in the meantime, to use masks."

How public health experts are responding

Some public health experts praised CDC's latest guidance as showing responsiveness to the evolving Covid-19 threat.

Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York, said that based on what scientists are learning about how the delta variant can cause breakthrough infections, "[t]his is a move in the right direction."

Separately, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical adviser to the White House, said the agency was right to re-evaluate its recommendations in light of new research. "I don't think you can say that this is just flip-flopping back and forth," Fauci said. "They're dealing with new information that the science is providing."

Other experts, however, criticized the new guidance as insufficient or potentially confusing.

For instance, Ali Mokdad, an epidemiologist at University of Washington and former CDC scientist, said the agency should have recommended universal masking. "The director said the guidance is for people in areas of high transmission, but if you look at the country, every state is seeing a rise in transmission," Mokdad said. "So why not say, 'Everybody in the U.S. should be wearing a mask indoors?' The whole country is on fire." (Collins, et al., CNN, 7/27; Slotnik, et al., New York Times, 7/27; Lovelace Jr., CNBC, 7/27; Abutaleb et al., Washington Post, 7/27; Banco/Cancryn, Politico, 7/27; Mandavilli, New York Times, 7/28)