As infections involving the new Delta variant of the COVID-19 virus continue to increase around the world, including in the U.S., health experts are yet again revisiting advice about who should wear masks and when.

On June 28, the Los Angeles County public health department advised all people, including those who are vaccinated, to wear masks in most indoor public settings, based on the fact that nearly half of the virus from cases in the county that were genetically sequenced now belong to the Delta variant. The variant, first identified in India, is far more contagious than previous strains of SARS-CoV-2, and could cause more severe disease. Then the World Health Organization reaffirmed its advice that vaccinated people continue to wear masks when in public settings as a precaution. The U.S. Centers for Disease Control (CDC) has still not changed its guidance for vaccinated people, last revised in May, which states that fully vaccinated individuals can resume most of their normal activities, without masks. CDC director Dr. Rochelle Walensky said in a broadcast interview that vaccines continue to protect against the Delta variant, and that it’s more important for unvaccinated people to wear masks to protect themselves from getting infected. But she did acknowledge that local and state policies could decide to be more stringent because of rising cases of infections with the Delta variant.
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That’s the case in L.A. county. Studies show that vaccines provide people with sufficient protection from getting sick with COVID-19, but even vaccinated people can still get infected and experience symptoms, albeit milder ones and very rarely. That’s why Los Angeles health officials issued the recommendation for all people, vaccinated or not, to wear masks indoors in grocery stores, theaters, workplaces and restaurants when not eating, since it’s hard to know whether other people in those environments are vaccinated. The advice comes just after the county had relaxed social restrictions and allowed restaurants, retail and entertainment facilities to open as more people were vaccinated.

Dr. Muntu Davis, health officer for L.A. County, says the decision is based on the continued uncertainty about how much vaccine immunity protects against the Delta variant. Early data from Israel suggest that even fully vaccinated people can become infected with the Delta variant, which isn’t entirely surprising since studies show that the vaccines are about 80{d847ba2f23daf15c773bda6cb71ac33aa9166b9578a171d654c6e3c528a0b6bc} effective in shielding people from getting COVID-19. Since there is a chance, even though it’s small, that vaccinated people can still get infected and potentially pass along the virus to unvaccinated individuals, asking vaccinated people to wear masks in public is “prudent,” says Davis. He says that in making the recommendation, officials considered the fact that four million people in the county are unvaccinated, and that by all accounts, vaccinated people who do get infected could experience no symptoms or mild symptoms and therefore might not know they can transmit the virus.

Local health officials across the country may soon be facing similar decisions. Nationally, nearly 70{d847ba2f23daf15c773bda6cb71ac33aa9166b9578a171d654c6e3c528a0b6bc} of adult Americans have had at least one shot, but that number is much smaller in some parts of the country, making those areas at higher risk for outbreaks involving the Delta variant.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief medical advisor to the White House, has said that the best way to slow or stop the variant is to vaccinate as much of the population as possible, as quickly as possible. Variants like Delta only emerge when the virus is reproducing and making copies of its genetic code; and it can only do this freely in someone with little immune defenses to counter infection—such as unvaccinated people.

The unvaccinated continue to be the most vulnerable to getting infected and becoming potentially severely ill with COVID-19, since the variant is highly contagious. Preliminary reports from Australia using close-circuit cameras that monitored how long people known to be infected had been in contact with people who eventually become infected suggest the Delta variant doesn’t need much time to hop from one person to another.

“The Delta variant is more infective, more contagious. So given that it is so contagious, and that there are wide swaths of the country that don’t even have 50{d847ba2f23daf15c773bda6cb71ac33aa9166b9578a171d654c6e3c528a0b6bc} of people vaccinated in the U.S., then I think that the mask recommendations are absolutely in line with how we’ve been approaching the pandemic from the get-go,” says Dr. Kirsten Lyke, professor of medicine at the University of Maryland School of Medicine who is leading some of the COVID-19 vaccine trials. “I personally err on the side of caution, and wear a mask indoors and out even though I’m vaccinated. I just think we’ve been surprised enough by COVID-19 that I’m not sure we fully understand the Delta variant, and the degree to which it is transmissible.”

Lyke says that vaccinated people can have “some degree of comfort” if they are unmasked and outdoors, as the CDC suggests, but that everyone, vaccinated or not, should “continue to be wary. There are micropockets of people who are not vaccinated. And the Delta variant is just going to roar through them.”

Vaccines and masks are an insurance policy for everyone, says Davis. “We think of vaccines like seatbelts. If you were in an accident, you’re less likely to have a serious outcome.” Given that there are millions of people still unvaccinated—and remaining uncertainties about how likely it is that a vaccinated person can spread the virus—the mask recommendation makes sense, he says, “while we are watching and learning more about the Delta variant. It’s not a requirement, but a recommendation for everyone to continue to wear masks in those settings where they don’t know if people are fully vaccinated or not.”

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