CDC Says New Mask Guidelines Informed by Cape Cod Outbreak

“High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus,” CDC Director

Rochelle Walensky

said Friday. “This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation.”

An internal CDC slide presentation earlier this week described the Delta variant as infectious as chickenpox, which used to sicken some four million Americans a year before a vaccine became available in the mid-1990s. The Delta variant is more transmissible than the common cold, seasonal flu and smallpox, but less contagious than measles, according to the presentation, which was first reported in the Washington Post.

“The war has changed,” the presentation said.

The presentation said the agency must improve its communications around breakthrough infections, or people being infected after receiving the vaccine. It also underscored the importance of vaccines in fighting Covid-19. Vaccination reduces the risk of severe disease and death from Covid-19 by at least 10-fold and reduces the risk of infection by threefold, the presentation said.

It isn’t clear how common breakthrough infections are and how much they contribute to the virus’s spread. Epidemiologists and physicians say that the current surge of infections and hospitalizations in the U.S. is largely being driven by transmission among unvaccinated people, and that areas with lower vaccine coverage have so far been hit harder.

The vaccines authorized for use in the U.S. were designed to prevent symptomatic disease from Covid-19; how well they protect against infection and transmission has been less clear. Data collected following their authorization suggested that vaccinated individuals who did get infected carried less virus and were less likely to transmit the virus, but those samples were taken before the Delta variant arrived in the U.S.

Dr. Walensky said Tuesday that everyone, including vaccinated people, should wear masks in indoor public spaces in areas where the Delta variant is spreading rapidly, and that everyone in K-12 schools should wear a mask this fall. Earlier in July, the CDC said only unvaccinated people in schools would need to wear masks.

The change in guidance has prompted local officials and employers across the country to reassess mask and vaccination policies.

Walmart Inc.

said it would reinstate a mask mandate for workers in U.S. counties deemed at high risk of Covid-19 transmission, and that it would encourage, but not require, shoppers to wear masks inside stores.

Amazon.com Inc.

said it would maintain its plan for some corporate employees to return to work in September, and that it wouldn’t require workers to get vaccinated.

Some public-health experts criticized the CDC this week for changing its mask guidance without releasing more scientific data in support of the new policy.

“The mistake is releasing the guidance without explanation,” said Vish Viswanath, a professor of health communication at the Harvard T.H. Chan School of Public Health. “One of the most important principles in communicating risk in such situations is complete transparency.”

In its internal presentation, the CDC pointed to studies that demonstrated that the Delta variant is associated with higher levels of the virus, a longer period of infectiousness and more severe disease than previous variants.

Among the 469 cases linked to the Barnstable outbreak in the CDC report, nearly 75% were fully vaccinated. For people with breakthrough infections, almost 80% had symptoms of cough, headache, sore throat or fever. Four were hospitalized and no deaths were reported, the CDC said. Infected people reported attending densely packed indoor events and outdoor events at bars, restaurants and houses.

It isn’t known how many people at the festivities might have been exposed or infected, or what proportion were vaccinated. Cases, especially those that were asymptomatic, were likely missed, and some people might not have gotten tested, said Mary Jo Trepka, an infectious disease epidemiologist at Florida International University. That makes it hard to interpret rates of infection and symptoms or apply them to the population at large.

“We only know about the people who got sick,” she said.

The report didn’t say whether, for people who became infected after vaccination, there was a connection between the amount of the virus and noticeable symptoms of disease.

Ashish Jha,

dean of the public-health school at Brown University, said that distinction had significant implications for policies like mask mandates.

“If you’re vaccinated and you get infected and you have a fever and you’re hacking up a lung and you have a sore throat—no one would question that in that case, you need to wear a mask,” Dr. Jha said. “The real question is, should asymptomatic vaccinated people be wearing mask to prevent spread to others? And I have not seen any data that supports that yet.”

Dr. Jha said he thinks this week’s guidance recommending masking in high-risk areas of the U.S. was reasonable, but also risked suggesting that vaccines aren’t effective against the Delta variant, which could discourage unvaccinated people from getting shots. The Biden administration and local officials across the country are working to persuade more Americans to get vaccinated after the rate of new inoculations tapered off in recent weeks.

“We have the tools to address this variant, and they’re called vaccines,” Dr. Jha said.

—Julie Wernau and Robbie Whelan contributed to this article.

Write to Brianna Abbott at brianna.abbott@wsj.com