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By any measure, the United States has some of the top public health experts in the world. Yet as the novel coronavirus began to spread early this year, these U.S. experts repeatedly recommended against a simple tactic to prevent spreading the infection: face masks.

The U.S. Centers for Disease Control and Prevention said in January that it did not recommend the use of masks for “people who are well.” On Feb. 29, U.S. Surgeon General Jerome Adams went further, tweeting a warning: “STOP BUYING MASKS.”

But weeks later, the advice was reversed. On April 3, as the number of deaths from the coronavirus in the United States surged to more than 7,000, the CDC altered its recommendation to state that “cloth face coverings” should be worn when social distance cannot be maintained.

Still, even as the changed policy was announced, President Trump said he would not personally be doing it. Some Republican lawmakers have mocked the practice of wearing masks, while local rules requiring masks have been rescinded in some locations following a backlash.

The U-turn regarding masks and the subsequent political divide over them has come to symbolize the chaos of the U.S. response to the still-raging pandemic. It also may be particularly damaging for America’s global standing, as it has drawn in not just political leaders like Trump but also widely respected public health experts who did not initially back the wearing of face masks.

“I’ve always thought of the CDC as a reliable and trusted source of information,” Siouxsie Wiles, an infectious-diseases specialist at the University of Auckland in New Zealand, told The Post last week. “Not anymore.”

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases and a key adviser on the pandemic in the early months, was asked at a House hearing this week whether he regretted the previous advice against masks.

Fauci said he did not regret the decision, arguing that at the time there was a “paucity of equipment” for front-line health workers.

The debate about masks could intensify in coming weeks as U.S. cases surge amid economic reopening. Some state governments are newly moving to mandate the wearing of masks in public, but plenty of people, including the president and some of his supporters, still rarely wear one.

In other nations, there is no such divide. In many Asian countries, surgical masks were socially acceptable long before the coronavirus pandemic, partly as a result of experience in past pandemics, like SARS in 2003.

“In Hong Kong, it is pretty common, even without an outbreak, to see people going around in masks because they may be sick and they don’t want to infect other people,” Keiji Fukuda, head of the University of Hong Kong’s School of Public Health, told Today’s WorldView in March.

As suspicion grew that asymptomatic cases were spreading the virus, many holdouts changed course. Singapore initially advised the general public against wearing masks but reversed that guidance, implementing a law on April 14 that imposed a $212 fine for those who flouted the rules.

Conducting high-quality research about the effectiveness of masks against the novel coronavirus is difficult during a pandemic, but a spate of studies released this summer supported mask-wearing.

One review funded by the World Health Organization and published in the Lancet journal looked at data from 172 observational studies and concluded that wearing face masks reduced the risk of coronavirus infection.

For many, this was all the evidence needed. Other policies for controlling the spread of the virus are enormously costly and practically difficult, from shutting down schools and offices to setting up complicated contact tracing. A mask, on the other hand, is cheap and has little downside.

And yet the U.S. president avoids wearing a mask in public, joining a small group of leaders that includes Mexico’s Andrés Manuel López Obrador and Belarus’s Alexander Lukashenko.

Brazilian President Jair Bolsonaro, who brushed off the threat of the virus as his country amassed the second-highest death toll in the world, was this week ordered by a Brazilian federal judge to wear a mask when in public in Brasília.

The divide over masks in the United States cannot be blamed on the president’s vanity alone, however. As Fauci noted on Tuesday, there were early U.S. concerns about the availability of surgical masks and N95 masks for essential workers, which had a distorting effect on the debate.

Supply concerns are also believed to have influenced the WHO’s belated decision to change its own global guidance in favor of masks since June 5, as some nations would not have been able to supply masks.

Other factors include the United States’ decentralized political system, as well as Americans’ focus on protecting individual liberty. “Making individual decisions is the American way,” Max Parsell, a 29-year-old power-line worker in Jacksonville, Fla., recently told The Post as he justified his decision not to wear a mask.

But the relentless toll of the pandemic on the United States, as well as the ensuing protests after the death of George Floyd, has left many outside nations reappraising the world’s lone superpower.

“We live with the idea that the U.S. has an ability to rebound that is almost unlimited,” Michel Duclos, a former French ambassador to Syria, told the Atlantic recently. “For the first time, I’m starting to have some doubts.”

The United States has faced serious blows to its international reputation before, notably after the 2003 invasion of Iraq and any number of decisions involving Trump since 2016. However, this time it isn’t just partisan political leaders who are facing hard questions, but also apolitical subject experts.

Bloomberg Opinion this week asked writers around the world what they thought of the U.S. response to the coronavirus, and several mentioned the politically charged debate over masks, as well as broader concerns about the way the United States uses the expertise it has built up.

“America has some of the very best professionals and hospitals in the world,” Ferninando Guigliano, an Italian columnist, explained. “But it lacks a centralized structure that gives you confidence that the country as a whole can counter the pandemic effectively.”

This story has been updated with details regarding the WHO’s guidance on masks.

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