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The use of masks to control the spread of the coronavirus has been divided geographically all along — with many Western countries late to recommend the use of face coverings already in wide use in East Asia.

But when the U.S. Centers for Disease Control and Prevention announced new guidance in May suggesting that vaccinated people no longer needed to wear masks, indoors or out, the United States joined a far smaller list of Western countries to move away from mask usage before the end of the pandemic, in the face of well-evidenced efficacy.

At the time, some international critics of the CDC said the agency seemed to have a knack for issuing the wrong advice at the wrong time, rebuking it for a decision that some experts said could rank alongside a mistake of the early pandemic — delaying a recommendation on masks until April 3, 2020, despite increasing evidence that the virus was being transmitted via airborne droplets.

“I’m constantly surprised at how quickly we’re willing to discard what we’ve learned,” said Ian Mackay, an associate professor of virology at the University of Queensland in Australia.

On Tuesday, less than three months later, the CDC announced it had again changed its guidance on masks, recommending once more that all Americans wear masks indoors in public spaces when in covid-19 hot spots.

In an interview with CNN on Wednesday, CDC Director Rochelle Walensky said the guidance was prompted by new data suggesting fully vaccinated people can sometimes pass on the delta variant.

“With prior variants, when people had these rare breakthrough infections, we didn’t see the capacity of them to spread the virus to others. But with the delta variant, we now see ... that you can actually now pass it to somebody else,” Walensky said.

Whiplash recommendations carry downsides. “Once you’ve let the genie out of the bottle, it’s really hard to put it back in,” said Lawrence Gostin, a professor of global health law at Georgetown University. Americans who have set aside their masks might not be persuaded to don them again.

Last year, the CDC was not alone in its skepticism. The World Health Organization did not publish global guidance supporting masks until June 5, more than two months after the United States did. The lack of mask availability early in the pandemic made recommending them difficult.

The WHO currently advises all people, vaccinated or not, to continue wearing masks when around others.

WHO spokesman Tarik Jasarevic said Tuesday that the global health body advises governments to continue emphasizing proven public health and social measures, including masks, “as long as there is community spread.”

There was community spread when the CDC issued its May 13 guidance. The United States that day recorded almost 50,000 new coronavirus cases. But vaccination rates were dipping. The message about masks appeared to be an attempt to incentivize the shots.

That gambit may have failed: The number of coronavirus vaccine doses given out this week across the United States is less than half of what it was the week of May 13. Many unvaccinated Americans appear to have ditched their masks anyway.

“The science was right,” Gostin said. “But the CDC is a public health organization. It’s not a scientific think tank. And if they spoke to any behavioral scientist at the time, they would have said that the message conveyed would be to everyone: Take off your mask.”

Christos Lynteris, a professor of medical anthropology at the University of St. Andrews in Scotland, said that “this is a major failure in public health communication, as ideally we should be aiming for masks to continue to be worn not simply for the duration of the pandemic but also after it.”

Few experts think that masks alone are a panacea. But many still believe they are a vital tool, alongside vaccination.

Masks are common during flu season in East Asian nations and were widely discussed as a factor in the early success of slowing the pandemic in some of those countries.

“Masking remains one of the most effective, safe and low-cost non-pharmaceutical interventions to reduce transmission,” said Sunny Wong, an associate professor at the Chinese University of Hong Kong.

Wong was the lead author on a study released in June 2020 finding that places that had adopted mask-wearing early in the pandemic, such as Hong Kong and Taiwan, by that point at least had milder outbreaks than others.

It was “reasonable” for governments to relax mask mandates earlier this year if they had high vaccination rates, Wong said, but only if there was low community transmission as well. “Otherwise, masks should remain a wise precaution, regardless of vaccination status,” Wong said.

Hong Kong, which is recording only a few new cases a day and has fully vaccinated roughly 30 percent of the population, is keeping masking recommendations in place, he noted.

Many governments recommend some form of masking, with only a few outliers in the West, including Sweden and Hungary.

Even at the recent Tokyo Olympics Opening Ceremonies, the global divide over masks was relatively muted. Most delegations wore masks, in line with local requirements — except for most of the athletes from Kyrgyzstan and Tajikistan, along with a few others.

The United States is not the only country to lift and then reinstate mask guidance. In Israel, an indoor mask mandate was lifted on June 15, only to be reimposed on June 25 as cases surged.

In South Korea, one of the first East Asian countries to chart a path out of the pandemic, the government announced in June that partially inoculated residents would soon be allowed to go mask-free outdoors.

But before the newly relaxed rules could go in place, the South Korean government canceled them in Seoul and neighboring regions, ordering even fully vaccinated residents to wear masks inside and outside.

In other nations such as France and Australia, regional rules on mask-wearing have returned this summer.

In England, mask rules were relaxed along with all other remaining restrictions on July 19, despite surging cases nationwide. The British government portrayed the move as “Freedom Day” and a return to normality.

Many government officials have said that despite the end of restrictions, masking may still be advisable.

Chris Whitty, England’s chief medical officer, said on July 7 that he would wear a mask “in any situation [that is] indoors and crowded, or indoors with proximity to other people,” even though that is not a government requirement and he is fully vaccinated.

Reinstating mask rules that have been lifted can be difficult. Israel has struggled with mask compliance since June 25.

Among further restrictions installed last week, the Israeli government can fine a business over $300 for serving somebody who is not wearing a mask or for not displaying a sign about the requirement of masks on the premises.

Despite these changes, masks are still divisive globally. Keiji Fukuda, head of the University of Hong Kong’s School of Public Health, said that an East-West divide on mask-wearing might persist.

“In Western countries, many of those opposed to wearing masks still appear to equate it to an infringement upon personal rights,” Fukuda said. “In Asia, and certainly in Hong Kong, wearing a mask is simply seen as a safe and prudent step to take, similar to hand-washing and staying away from others if you are sick.”

However, Fukuda, a former senior official with both the CDC and the WHO, said the global divide on masks appeared to be lessening.

“If nothing else, I hope most people have come to learn that dealing with covid is not simple and requires a mix of science, political judgment, really good communications, trust and flexibility,” he said. “I might also add humility.”

This report has been updated.