CDC Says New Mask Guidelines Informed by Cape Cod Outbreak

CDC Says New Mask Guidelines Informed by Cape Cod Outbreak

18:54 - Vaccinated people infected by Delta variant appeared to carry as much virus as those in unvaccinated cases

A Covid-19 outbreak on Cape Cod helped prompt the Centers for Disease Control and Prevention to urge some Americans to start wearing masks again, the agency said, because it demonstrated that vaccinated people infected with the Delta variant might be as contagious as those who are unvaccinated.

The CDC said in a report on Friday that 127 vaccinated people infected with the Delta variant during the outbreak appeared to carry as much virus as 84 unvaccinated or partially vaccinated people who became infected. The report referred to an outbreak in Barnstable County, Mass. Local officials there have said that at least 430 confirmed Covid-19 cases have been linked to one cluster following festivities over the July 4 weekend in Provincetown, on the tip of Cape Cod.

“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 just how common breakthrough infections are and how much they contribute to the virus’s spread. The CDC stopped tracking all reported breakthrough cases that don’t lead to hospitalization or death in April. Dr. Walensky said Tuesday that the agency is studying breakthrough infections at sites across the country.

Epidemiologists and physicians say the current wave of infections and hospitalizations in the U.S. is largely being driven by transmission among unvaccinated people, and 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. Data suggest that they continue to largely protect people from the worst consequences of Covid-19. The CDC has said virtually all people hospitalized with Covid-19 in recent weeks have been unvaccinated.

How well the vaccines 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. Most said they support the guidance itself in light of the recent surge and the heightened contagiousness of the Delta variant.

“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 is unclear how many people at the festivities might have been exposed 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. We don’t know about the people who were exposed,” she said.

The Massachusetts Department of Public Health on July 3 reported a two-week average of zero cases per 100,000 residents of the town in Barnstable County, according to the CDC report. By July 17, that incidence had increased to 177 daily cases per 100,000 town residents. The CDC said this week that it considered 50 to 100 daily cases per 100,000 people to be a rate of spread substantial enough to warrant vaccinated people wearing masks in public indoor spaces.

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.

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