Throughout the pandemic, masks were among the most controversial public health measures in the United States, symbolizing a bitter partisan divide over the role of government and individual freedoms.

Now, with a new variant of the coronavirus spreading rapidly around the world, masks are once again the focus of conflicting views and fears about how the pandemic will unfold and the constraints needed to cope with it.

The renewed concerns follow forest fire growth of the Delta variant, a highly infectious form of the virus first discovered 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 fully vaccinated people no longer need to mask themselves, even indoors. The council marked a fundamental change in American life and set the stage for a national reopening that continues to gain momentum.

But that was before the delta variant spread. Concerned about a global surge in cases, the World Health Organization reiterated its long-standing recommendation last week that everyone – including those who have been vaccinated – wear masks to contain the spread of the virus.

Los Angeles County health officials followed on Monday, recommending that “everyone, regardless of vaccination status, should wear masks as a precaution in public places indoors.”

Barbara Ferrer, the county’s public health director, said the new recommendation was because of the increase in infections, an increase in cases due to the worrying Delta variant, and the continued high numbers of unvaccinated residents, especially children, black and Latin American residents, and important workers.

About half of Los Angeles County’s residents are fully vaccinated, and about 60 percent have received at least one dose. While the number of positive tests in the county is still below 1 percent, the rate has increased, added Dr. Ferrer added, and the number of reinfections in residents who were previously infected and not vaccinated has increased.

As far as Los Angeles County has managed to control the pandemic, it was due to a multi-faceted strategy that combined vaccinations with health restrictions to curb new infections, said Dr. Ferrer. Natural immunity among those already infected has also kept transmission low, she noted, but it is not clear how long the natural immunity will last.

“We don’t want to go back to lockdown or disruptive mandates here,” said Dr. Ferrer. “We want to stay on the path we are currently on, which keeps the transmission by the community very low.”

Health officials in Chicago and New York City said Tuesday that they had no plans to re-examine masking requirements. Officials from the Centers for Disease Control and Prevention declined to comment but did not signal any intention to revise or re-examine the masking recommendations for fully vaccinated individuals.

“When the CDC made the recommendation To stop masking, it didn’t anticipate that we might be in a situation where we might need to recommend masking again, ”said Angela Rasmussen, researcher at the Vaccines and Infectious Disease Organization at the University of Saskatchewan in Canada .

“Nobody will want to do it. The people understandably accuse them of having moved the goal posts. “

But the Delta variant’s trajectory outside of the United States suggests that concerns are likely to increase.

Even Israel – which has one of the highest vaccination rates in the world and aggressively immunizes young adolescents and teenagers who qualify – has reintroduced the mask requirement in indoor public spaces and at large outdoor public gatherings after hundreds of new Covid-19 cases were discovered in the past few days, including in people who received both doses of the Pfizer BioNTech vaccine.

This isn’t the first time the world has been consumed by a more contagious variant of the coronavirus. The alpha variant rolled over the UK and brought the rest of Europe to a standstill earlier this year. Alpha quickly became the dominant variety in the United States by late March, but the rapid pace of vaccination slowed its spread and saved the nation a huge surge in infections.

But Delta is considered even more terrifying. Much of what is known about the variant is based on its distribution in India and the UK, but early evidence suggests it is perhaps twice as contagious as the original virus and at least 20 percent more contagious than Alpha.

Updated

June 29, 2021, 10:46 p.m. ET

In many Indian states and European nations, Delta has quickly overtaken Alpha and has become the dominant version of the virus. It is well on its way to do the same in the United States.

Among the many mutations in the variant are some that can help the virus to partially evade the immune system. Several studies have shown that while the current vaccines are effective against Delta, they are slightly less effective than most other variants. In people who received only one dose of a two-dose regimen, protection against the variant is significantly reduced compared to effectiveness against other forms of the virus.

The WHO rationale for keeping masking is that while vaccines are very effective at preventing serious illness and death, it is not known to what extent vaccines prevent mild or asymptomatic infections. (CDC officials disagree and say the risk is minimal.)

The WHO claims that vaccinated people should wear masks in crowded, narrow and poorly ventilated areas and take other preventive measures like social distancing.

“What we are saying is, ‘Once you are fully vaccinated, keep playing it safe because you could end up being part of a chain of transmission. You may not be fully protected, ‘”said Dr. Bruce Aylward, a senior adviser to WHO, at a news conference last week.

Even in countries with relatively high vaccination rates, there has been an increase in infections from the delta variant. Great Britain, where around two-thirds of the population have received at least one dose of the Pfizer-BioNTech or AstraZeneca vaccine and almost half two doses, is still struggling with a sharp increase in infections from the variant.

It is not certain which course the delta variant will take in the USA. The coronavirus infections have been falling for months, as have been hospital admissions and deaths. But dr. Anthony S. Fauci, the nation’s leading infectious disease doctor, has described the variant as “the greatest threat” to eliminating the virus in the United States.

When CDC officials lifted masking recommendations in May, they cited research showing that fully vaccinated individuals are unlikely to become infected with the virus, even with asymptomatic infections.

But the partial immune evasion variant’s talent makes researchers nervous, as it suggests that fully vaccinated people sometimes get asymptomatic infections and unwittingly pass the virus on to others, even if they never get the disease.

The Delta variant can infect people who have been vaccinated, although its ability to do so is very limited, said Bill Hanage, an epidemiologist at Harvard TH Chan School of Public Health. “If you’re in a fall-climbing place, 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 do not recommend that fully vaccinated people always wear masks indoors, but some are now suggesting that this may be appropriate depending on local circumstances – for example, anywhere the virus is circulating in high numbers or vaccination rates are very low.

“Masking in closed public spaces must continue after vaccination until we can all be vaccinated or get 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, around half of Americans are not vaccinated, and much of the country remains vulnerable to outbreaks of the virus and its variants. Vaccinations for children under the age of 12 are expected to be approved in autumn at the earliest.

In Saskatchewan, Canada, the reopening took place in stages tied to the vaccination rates of the population and the percentage of people vaccinated in specific age groups.

The province moves to step 3 of re-entry on July 11, but can maintain indoor mask requirements and congregation size restrictions, said Dr. Rasmussen from the University of Saskatchewan. The strategy “makes a lot more sense than just saying, ‘When you are fully vaccinated, take off your mask,'” she said.

However, some scientists fear that it will be nearly impossible to reintroduce masking requirements and other precautions, even in places where it might be a good idea.

“It’s hard to get that back,” said David Michaels, an epidemiologist and professor at the George Washington School of Public Health, referring to the CDC advice. But with the advent of the delta variant, it is also “extremely dangerous to continue the cultural norm that nobody wears a mask”.

Dr. Ezekiel Emanuel, vice president of global initiative at the University of Pennsylvania, said introducing the variant should lead to a reconsideration of the mask requirement.

He still wears a mask in public places like grocery stores and even on crowded sidewalks. “We don’t even know the long-term consequences of a slight infection,” he said, referring to so-called long Covid. “Is it worth a little more insurance by wearing a mask? Yes.”

Monroe Harmon, 60, had coffee outside the Whole Foods Market in downtown Los Angeles Tuesday morning and said he thought a step back on masking requirements for everyone is a good idea.

“There are so many people who say they just want their lives back,” said Mr. Harmon, who works for a security company. “I think you kind of roll the dice if you decide, ‘I want my life back, I won’t wear a mask, I won’t distance myself.'”

Jill Cowan and Ana Facio-Krajcer contributed the coverage from Los Angeles.