Los Angeles County and the W.H.O. warned that even immunized people should wear masks indoors. Some scientists agreed, but urged a localized approach.Throughout the pandemic, masks have ranked among the most contentious public health measures in the United States, symbolizing a bitter partisan divide over the role of government and individual liberties.Now, with a new variant of the coronavirus rapidly spreading across the globe, masks are again the focus of conflicting views, and fears, about the course of pandemic and the restrictions required to manage it.The renewed concerns follow the wildfire growth of the Delta variant, a highly infectious form of the virus first detected in India and later identified in at least 85 countries. It now accounts for one in five infections in the United States.In May, federal health officials said that fully vaccinated people no longer needed to mask up, even indoors. The advice signified a sea change in American life, setting the stage for a national reopening that continues to gain momentum.But that was before the spread of the Delta variant. Worried by a global surge in cases, the World Health Organization last week reiterated its longstanding recommendation that everyone — including the inoculated — wear masks to stem the spread of the virus.On Monday, health officials in Los Angeles County followed suit, recommending that “everyone, regardless of vaccination status, wear masks indoors in public places as a precautionary measure.”Barbara Ferrer, the county’s public health director, said the new recommendation was needed because of upticks in infections, a rise in cases due to the worrisome Delta variant, and persistently high numbers of unvaccinated residents, particularly children, Black and Latino residents and essential workers.Roughly half of Los Angeles County residents are fully vaccinated, and about 60 percent have had at least one dose. While the number of positive tests is still below 1 percent in the county, the rate has been inching up, Dr. Ferrer added, and there has been a rise in the number of reinfections among residents who were infected before and did not get vaccinated.To the extent that Los Angeles County has managed to control the pandemic, it has been because of a multilayered strategy that combined vaccinations with health restrictions aimed at curbing new infections, Dr. Ferrer said. Natural immunity among those already infected has also kept transmission low, she noted, but it is not clear how long natural immunity will last.“We don’t want to return to lockdown or more disruptive mandates here,” Dr. Ferrer said. “We want to stay on the path we’re on right now, which is keeping community transmission really low.”A Covid-19 memorial wall in London, where infections with the Delta variant are rising fast. Andy Rain/EPA, via ShutterstockHealth officials in Chicago and New York City said on Tuesday that they had no plans to revisit mask requirements. Officials at the Centers for Disease Control and Prevention declined to comment, but have not signaled any intention to revise or re-examine masking recommendations for those who are fully vaccinated.“When the C.D.C. made the recommendation to quit masking, it didn’t anticipate being in a situation where we might need to recommend masking again,” said Angela Rasmussen, a research scientist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada.“Nobody’s going to want to do it. People are understandably accusing them of moving the goalposts.”But the Delta variant’s trajectory outside the United States suggests that concerns are likely to intensify.Countries in the Asia-Pacific region are now reimposing restrictions and stay-at-home orders as the variant drives new surges. Four Australian cites have reimposed lockdowns, and on Monday, the Malaysian government said nationwide stay-at-home orders would be extended indefinitely.Even Israel — which has one of the highest vaccination rates in the world and is aggressively immunizing younger adolescents and teenagers who qualify — has reinstated masking requirements in public indoor spaces and at large public gatherings outdoors, after hundreds of new Covid-19 cases were detected in recent days, including among people who had received both doses of the Pfizer-BioNTech vaccine.This is not the first time the world has been consumed by a more contagious variant of the coronavirus. The Alpha variant walloped Britain and brought the rest of Europe to a standstill earlier this year. In the United States, Alpha quickly became the dominant variant by late March, but the rapid pace of vaccination blunted its spread, sparing the nation a big surge in infections.But Delta is thought to be even more fearsome. Much of what is known about the variant is based on its spread in India and Britain, but early evidence indicates that it is perhaps twice as contagious as the original virus and at least 20 percent more contagious than Alpha.In many Indian states and European nations, Delta quickly outpaced Alpha to become the dominant version of the virus. It is on track to do the same in the United States.Among the variant’s many mutations are some that may help the virus partly dodge the immune system. Several studies have shown that while the current vaccines are effective against Delta, they are slightly less so than against most other variants. For individuals who have received only one dose of a two-dose regimen, protection against the variant is significantly reduced, compared with efficacy against other forms of the virus.A drive-through vaccination site in North Sumatra, Indonesia, on Tuesday.Dedi Sinuhaji/EPA, via ShutterstockThe W.H.O.’s rationale for maintaining masking is that while immunization is highly effective at preventing severe illness and death, the degree to which vaccines prevent mild or asymptomatic infections is unknown. (Officials at the C.D.C. disagree, saying the risk is minimal.)The W.H.O. maintains that vaccinated people should wear masks in crowded, close and poorly ventilated areas, and should continue with other preventive measures, like social distancing.“What we’re saying is: ‘Once you’ve been fully vaccinated, continue to play it safe, because you could end up as part of a transmission chain. You may not actually be fully protected,’” Dr. Bruce Aylward, a senior adviser to the W.H.O., said at a news briefing last week.Even countries with relatively high vaccination rates have seen an increase in infections driven by the Delta variant. Britain, where some two-thirds of the population have received at least one dose of the Pfizer-BioNTech or AstraZeneca vaccine and just under half have received two doses, is nonetheless grappling with a sharp rise in infections from the variant.It is not certain what course the Delta variant will take in the United States. Coronavirus infections have been plunging for months, as have hospitalizations and deaths. But Dr. Anthony S. Fauci, the nation’s top infectious disease doctor, has called the variant “the greatest threat” to eliminating the virus in the United States.In May, when C.D.C. officials lifted masking recommendations, they cited research showing that fully vaccinated people were unlikely to become infected with the virus, even with asymptomatic infections.But the variant’s talent for even partial immune evasion makes researchers nervous, as it suggests that fully vaccinated people may sometimes pick up asymptomatic infections and unknowingly spread the virus to others even if they never become ill.The Delta variant can infect vaccinated people, although its ability to do so is very limited, said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “If you are in a place where cases are climbing, wearing a mask indoors in crowded public spaces is a way to keep yourself from contributing to the spread of Delta,” he said.Other scientists stop short of recommending that fully vaccinated people always wear masks indoors, but some now suggest that this may be appropriate depending on local circumstances — for example, wherever the virus is circulating in high numbers, or vaccination rates are very low.“Masking in public enclosed spaces needs to continue even after vaccination, until we can get everyone vaccinated or a new vaccine that is more effective against Delta transmission,” said Dr. Ravindra Gupta, a virologist at the University of Cambridge in the United Kingdom.Even now, roughly half of Americans are not vaccinated, and a wide swath of the country remains vulnerable to outbreaks of the virus and its variants. Vaccines for children under age 12 are not likely to be authorized until the fall, at the earliest.Streets in Sydney, Australia, emptied after a two-week lockdown to contain an outbreak of the Delta variant.Saeed Khan/Agence France-Presse — Getty ImagesIn Saskatchewan, Canada, reopening has proceeded in stages that are tied to population vaccination rates and the percentage of people in certain age brackets who have been vaccinated.The province is moving to Step 3 of re-entry on July 11, but may maintain indoor masking requirements and restrictions on the size of gatherings, said Dr. Rasmussen, of the University of Saskatchewan. The strategy “makes a lot more sense than just saying, ‘If you’re fully vaccinated, go ahead and take off your mask,’” she said.Yet some scientists fear it will be nearly impossible to reimpose mask mandates and other precautions, even in places where it may be a good idea to do so.“It’s difficult to walk that back,” said David Michaels, an epidemiologist and professor at the George Washington School of Public Health, referring to the C.D.C. advice. Yet with the rise of the Delta variant, it also is “extremely dangerous to continue the cultural norm of no one wearing a mask.”Dr. Ezekiel Emanuel, vice president for global initiative at the University of Pennsylvania, said the arrival of the variant should prompt a rethinking of mask mandates.He still wears a mask indoors in public places like grocery stores, and even on crowded city sidewalks. “We don’t know the long-term consequences of even a mild infection,” he said, referring to so-called long Covid. “Is a little more insurance from wearing a mask worth it? Yes.”Sipping coffee outside the Whole Foods Market in downtown Los Angeles on Tuesday morning, Monroe Harmon, 60, said he thought a step back toward masking requirements for everyone might be a good idea.“There’s so many people suggesting that they just want their lives back,” said Mr. Harmon, who works for a security company. “I think you kind of roll the dice when you decide, ‘I want my life back, I’m not going to wear the mask, I’m not going to distance.’”Jill Cowan and Ana Facio-Krajcer contributed reporting from Los Angeles.