To mask or not to mask?

The rise of coronavirus’s more-infectious Delta variant is prompting new questions about mask-wearing, even for vaccinated people. And conflicting public health advice has left many people scratching their heads.

The World Health Organization recently reiterated its world-wide guidance that everyone should wear a mask indoors. And Los Angeles County on Monday strongly recommended the same, citing the Delta variant. But the CDC Wednesday reaffirmed its guidance that vaccinated people don’t need to wear masks in most settings.

So what to make of this and what’s the safest thing to do to protect against the spread of variants? We asked doctors.

Why are public health officials talking about wearing masks again?

WHO officials have repeated their recommendation that everyone, vaccinated or not, continue wearing masks indoors as a precautionary measure. That message came on the heels of the rapid spread of the so-called Delta variant, a more-infectious version of the coronavirus that was first identified in India in late 2020 and has, according to the WHO, spread to at least 85 countries, including the U.S.

The Los Angeles County department of public health has also recommended that everyone—including people who are fully vaccinated—wear masks indoors in public places. The recommendation came as nearly half of cases sequenced in the county were due to the Delta variant.

Masked and unmasked people make their way through a market in Los Angeles. The county is seeing more cases of the Delta variant.

Masked and unmasked people make their way through a market in Los Angeles. The county is seeing more cases of the Delta variant.

Photo: frederic j. brown/Agence France-Presse/Getty Images

What does the CDC say in the U.S.?

The CDC surprised many in May when it said fully vaccinated people didn’t need to wear masks indoors in most settings unless state, local or business rules require them to. (It does continue to recommend mask-wearing for everyone while traveling by plane, bus or train, and in high-risk settings such as hospitals, nursing homes and prisons.)

The agency hasn’t revised those recommendations as the Delta variant has spread. On Wednesday, CDC Director Rochelle Walensky reiterated that guidance, saying that full vaccination protects Americans from variants, including Delta. But she also said that local governments should make policies for their communities depending on their circumstances.

Earlier

People who are fully vaccinated don’t need to wear a mask or physically distance for most outdoor or indoor activities, the Centers for Disease Control and Prevention’s Dr. Rochelle Walensky said Thursday, but some exceptions remain, including the requirements for public transportation. Photo: Justin Lane/Shutterstock (Video from 5/13/21) The Wall Street Journal Interactive Edition

What should I make of these statements?

The WHO’s message isn’t new. The global health agency has long recommended that everyone—vaccinated or not—continue wearing masks indoors.

Most experts say the WHO’s global message is prudent given low vaccination rates and high Covid-19 rates in many countries. Although even Israel, a country with high vaccination rates and relatively low Covid cases, has reimposed some precautions after the Delta variant infected some vaccinated people.

It’s important to note that the WHO “makes recommendations for the whole planet,” says David Wohl, a professor of medicine in the division of infectious diseases at University of North Carolina at Chapel Hill. Although Delta-variant cases are rising in the U.S., the U.S. also has higher vaccination rates than many other countries. “We are in a very different situation than most of the world,” he says.

I’m fully vaccinated and my local government hasn’t changed its recommendations. Should I wear a mask when indoors?

Community transmission and vaccination rates are an important consideration, scientists say. Much of the United States has very low transmission rates now—but there are pockets where rates are rising.

“It’s very much region-dependent,” says Paul E. Sax, clinical director of the division of infectious diseases at Brigham and Women’s Hospital in Boston. Some states in the South, for instance, have low vaccination rates, while many Northeastern states have higher rates.

Even more important is to look at a community’s hospitalization rates for Covid-19, says Panagis Galiatsatos, a pulmonary and critical care physician at Johns Hopkins Health System in Baltimore. “If your hospitals are overwhelmed, that’s a good surrogate for low vaccination rates and probably a large denominator of Covid cases,” says Dr. Galiatsatos.

People receive Covid-19 vaccine doses in Los Angeles. The U.S. is trying to vaccinate more people as the Delta variant spreads.

People receive Covid-19 vaccine doses in Los Angeles. The U.S. is trying to vaccinate more people as the Delta variant spreads.

Photo: Ringo Chiu/Zuma Press

So when should fully vaccinated people wear masks indoors?

Consider community transmission and hospitalization rates, how much you know about other people’s vaccination status, how crowded the setting is—and your own tolerance for risk.

Monica Gandhi, an infectious disease physician and professor of medicine at the University of California, San Francisco, says if the seven-day average of positive Covid-19 tests in your community reaches what the CDC deems a “substantial” level—defined as 8% or more—fully vaccinated adults may want to consider wearing masks inside when in public settings.

Some doctors take a more conservative approach than the CDC’s guidance. Dr. Sax says any time you’re somewhere where people’s vaccination status is unknown, such as in a store, it’s a good idea to wear a mask.

Dr. Wohl says the decision to wear a mask indoors as a fully vaccinated person is based on one’s comfort level with a certain level of risk. But he recommends wearing a mask in most public indoor settings and being extra cautious in higher-risk settings such as restaurants and gyms. “I feel a little bit uneasy eating indoors unless there’s a good amount of distance between tables and the servers are wearing masks,” he says.

Are there any groups of vaccinated people who should be more vigilant than others about masking indoors in public settings?

Yes. Elderly people, those with compromised immune systems and people with health conditions that put them at risk of developing a more serious case of Covid-19 should be more cautious, says Gregory Poland, director of the Mayo Vaccine Research Group. “I think they should be particularly attentive to masking indoors because their risk is higher,” says Dr. Poland.

I’m going to a party and everyone there will be fully vaccinated. Do I have to wear a mask?

Doctors say if you’re confident that the people there are indeed fully vaccinated then you can safely be indoors without a mask.

What about adults who are not vaccinated?

Experts agree that if you’re not vaccinated, you should mask when inside in a public setting and in private homes.

Do I need to wear a mask outdoors?

Scientists say that in most outdoor settings, you don’t need to wear a mask. But if you’re in a crowded situation and in close proximity to others, you may want to consider wearing a mask, some doctors say. “Certainly this Fourth of July even outdoors in a highly crowded situation, I’d be in a mask,” says Dr. Poland.

Write to Sumathi Reddy at sumathi.reddy@wsj.com