U.S. public health officials’ guidance on mask-wearing is evolving because the virus behind COVID-19 is evolving—and flexible tactics are needed to fight a pandemic that is clearly far from over.
Less than three months after the Centers for Disease Control and Prevention had said that vaccinated people need not mask indoors or outdoors, based on the high protection conferred by the three COVID-19 vaccines authorized in this country, the CDC updated its guidance, urging everyone regardless of vaccination status to wear masks in schools and in public indoor spaces, in parts of the country with high or substantial coronavirus transmission.
Behind the stark warning were even starker facts: As SARS-CoV-2 has spread around the world and replicated in hundreds of millions of people, the virus has had more than a year to stumble upon mutations that let it more effectively enter human cells and evade our immune systems. Now, the world is facing the Delta variant first identified in India, which is roughly twice as transmissible as earlier strains of the virus.
The CDC’s latest guidance rests in part on a newly published analysis of a COVID-19 outbreak in Massachusetts, which began in Cape Cod after gatherings over the July 4 weekend. Of the outbreak’s 469 documented cases, 346—about three-fourths—were among people who had been vaccinated. Of these vaccinated cases, about 80 percent showed symptoms. Delta seems to have driven the outbreak: Health officials sequenced the virus in 133 of the patients, and 90 percent of them had the Delta variant.
Crucially, vaccination seems to have staved off severe outcomes: The CDC reports that within the Massachusetts outbreak, only five people had been hospitalized as of July 27, and none had died.
But this outbreak gives early evidence that for Delta infections, viral loads among the vaccinated and unvaccinated may resemble each other. That means it’s now possible vaccinated people who get even mild infections of Delta can transmit the virus to others—which could put people who can’t get the vaccines or don’t get their full efficacy, such as those with compromised immune systems, at even greater risk.
The global rise of Delta “was probably avoidable, but that [would have taken] a more unified and coordinated response—not just here in the United States, but elsewhere,” says University of Pittsburgh microbiologist Vaughn Cooper. “We are connected, and this variant is remarkably contagious, and our vaccination levels are just not uniformly high.”
How does the CDC measure viral load?
Less than half of the U.S. population is fully vaccinated, according to the CDC, and vaccination rates today are less than a fifth of what they were in April—a lag that has fueled new outbreaks in undervaccinated pockets of the country. In addition to the CDC's new guidance, President Biden announced earlier this week that his administration is considering requiring all federal workers to be vaccinated against COVID-19. Biden’s remarks followed the the U.S. Department of Veterans Affairs mandating vaccines for its 115,000 frontline healthcare workers.
Though most COVID-19 transmission in the U.S. is among people who are unvaccinated, in cases when vaccinated people suffer so-called breakthrough infections from the Delta variant, those people likely can transmit the SARS-CoV-2 virus, experts say.
In a July 27 news conference, CDC Director Rochelle Walensky noted that people with breakthrough infections carry similar numbers of viral particles as those detected in unvaccinated patients. And on July 30, the CDC published some of the preliminary data backing up that conclusion.
The newly released data includes what’s called a cycle threshold (Ct) value. The most sensitive SARS-CoV-2 tests rely on a technique called PCR, which uses multiple replication cycles to amplify a sample’s snippets of viral genetic material to detectable levels. In general, the fewer cycles it takes to get a clear positive signal, the more viral genetic material the sample started with, and the lower the Ct value.
Among 211 people infected in the Cape Cod outbreak, the average Ct values were indistinguishable between 127 vaccinated people and 84 people who were unvaccinated, not fully vaccinated, or with unknown vaccination status. But Ct values can vary based on how samples were collected, stored, and transported, and because of that variation, there’s no universally accepted conversion between a given Ct value and a specific amount of virus present in the sample.
The similarity seen in the Cape Cod data “might mean that the viral load of vaccinated and unvaccinated persons infected with SARS-CoV-2 is also similar,” the CDC report says. “However, microbiological studies are required to confirm these findings.”
Massachusetts state senator Julian Cyr, who represents Cape Cod and is on the local county’s COVID-19 task force, notes that the outbreak is scientifically useful because Provincetown health officials did such a thorough job tracking it from the beginning.
“We’re very much on the cutting edge of science because we have such good data,” Cyr says. “Hopefully, that’s going to really inform public policy going forward, as we’re trying to figure out what to do in the context of Delta.”
The idea that Delta is especially potent fits with a July 9 study that has not yet been peer-reviewed, which shows that people infected with the Delta variant have about a thousand times more viral particles in their respiratory tracts when they test positive for COVID-19 than people with previous strains at the same stage of infection. (Read more about why breakthrough infections happen and how rare they are today.)
“The Delta variant is showing every day its willingness to outsmart us and to be an opportunist in areas where we have not shown a fortified response against it,” Walensky said in the July 27 briefing.
Where masks are most needed in the U.S.
Many public health experts had lamented the May guidance to relax masking in public places, worried that without verifying who is vaccinated, the change in approach would allow unvaccinated Americans to eschew masks—and place others at risk for new, emerging variants. (Find out why scientists have been urging for local mask mandates for months.)
The Delta variant, which has been identified in 111 countries so far, has made the situation that much worse. Delta makes up more than 80 percent of new cases in 39 of those countries, including the U.S., the United Kingdom, India, Germany, and Denmark, and is continuing to spread.
“I don’t think we have digested that this variant is kind of a quantum leap ahead in transmissibility,” Cooper says.
In a recent study in the New England Journal of Medicine, a team of U.K. researchers found that among people who had received the full two-dose regimens of the Oxford-AstraZeneca or Pfizer-BioNTech vaccines, the shots were slightly less effective against Delta than against previous variants. But they were still 75 to 88 percent effective at preventing symptomatic COVID-19.
With the Delta variant spreading rapidly, public health experts agree that extra forms of protection—especially “non-pharmaceutical interventions” such as masks—are needed to bring down current outbreaks and buy time for newly vaccinated people to reach full immunity, especially in parts of the country where vaccination rates are low or infections are high. People are considered immune two weeks after their second doses of the Pfizer-BioNTech and Moderna vaccines, and two weeks after the single-dose Johnson & Johnson shot.
The CDC defines places with “substantial” transmission levels as states and counties where there are between 50 and 99.9 new cases per 100,000 people in the past seven days. “High” transmission regions are where there are 100 or more new cases per 100,000 in the same time frame.
Twenty U.S. states, including Arkansas, Florida, and Louisiana, have “high” transmission as of July 29. And more than two out of every three U.S. counties have substantial or higher levels of community transmission, thereby falling under the CDC’s new guidance. More than half of all U.S. counties—mostly concentrated in the South—fall into the high-transmission category, according to the CDC.
The CDC is also suggesting that vaccinated people in any part of the country who live with young children or people with compromised immune systems don masks when in public spaces indoors. The leaked CDC document notes that “universal masking is essential to reduce transmission of the Delta variant.”
The renewed focus on masking is “great news, it’s very welcome,” says epidemiologist Dominique Heinke, a North Carolina-based graduate of Harvard University’s T.H. Chan School of Public Health. Vaccines remain a vital tool for combating the virus, she says, but face masks not only can help stanch its spread—they can also help prevent its continued mutation.
“It’s hard for us to stay on guard all the time, and so we all are kind of like, It’s over, we can relax, but viruses don’t think that way,” Heinke adds. “They’re a little bit like zombies in a movie. You think you’ve protected yourself, and then they just come ripping through the door.”
Will masks become mandatory?
As concerns over Delta and the U.S.’s vaccination slowdown have mounted, scientists and public health experts have increasingly called for communities to reinstate mask requirements.
National Nurses United, the largest national organization of registered nurses, sent a letter to Walensky on July 12 calling for the agency to reconsider their position on dropping the mask mandate. "New variants, such as the Delta Plus and Lambda variants, have and will continue to emerge and pose a threat while the virus continues to spread," they wrote.
Some U.S. counties heeded the call before the CDC’s reversal, including Los Angeles County, which now requires masks for everyone in indoor spaces. On July 23, St. Louis, Missouri, and the broader St. Louis County also announced a joint mask mandate for indoor public places and public transportation.
In other parts of the country, public health officials have focused their funds and attention on vaccination outreach. To date, seven U.S. states have banned local districts from mandating masks in schools—even among students younger than 12 who cannot yet be vaccinated.
Local mandates are also facing politically charged blowback: On July 26, Missouri’s attorney general sued St. Louis over its new mask mandate, arguing that the move was “continued government overreach,” as well as “unacceptable and unconstitutional.” On July 27, St. Louis County voted to rescind its mask mandate.
“There is a real COVID fatigue, so it’s harder to sell [mask-wearing] today than it was a year ago,” says George Turabelidze, the state epidemiologist for Missouri, in an interview with National Geographic before the St. Louis mask announcements. “We do not anticipate any improvement anytime soon.”
In the CDC’s latest briefing, Walensky stressed how seriously the agency is taking the threat of Delta—and the policy changes the variant is forcing them to make.
“Eighteen months through this pandemic, not only are people tired, they’re frustrated. We have mental health challenges in this country. We have a lot of continued sickness and death in this country. Our health systems are, in some places, completely overrun [by] what is preventable,” she said. “I know that, in the context of all that, it is not a welcome piece of news that masking is going to be part of people’s lives who are already vaccinated.
“This new data weighs on me. This new guidance weighs on me,” she added. “This was not a decision that was taken lightly.”
Editor's Note: This story was updated on July 30 to include details about the COVID-19 outbreak in Massachusetts that influenced the CDC's new mask guidance.