The World Health Organization (WHO) on Friday announced changes to its guidelines on who should wear a mask during the Covid-19 pandemic and where they should wear it.

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The new guidance recommends that the general public wear cloth masks made from at least three layers of fabric “on public transport, in shops, or in other confined or crowded environments.” It also says people over 60 or with preexisting conditions should wear medical masks in areas where there’s community transmission of the coronavirus and physical distancing is impossible, and that all workers in clinical settings should wear medical masks in areas with widespread transmission.

It’s a major update to the agency’s April 6 recommendations, which said members of the general public “only need to wear a mask if you are taking care of a person with Covid-19” or “if you are coughing or sneezing.” And it’s important advice for countries around the world battling the virus, especially those in South America, the Middle East, and Africa, where the rate of Covid-19 transmission appears to be accelerating.

At a WHO press conference on June 3, Michael Ryan, an infectious disease epidemiologist and the executive director of the WHO’s Health Emergencies Programme, said WHO still believes that masks should primarily be used “for purposes of source control — in other words, for people who may be infectious, reducing the chances that they will infect someone else.”

And on Friday, WHO Director-General Tedros Adhanom Ghebreyesus offered a few words of warning as part of the announcement: “Masks can also create a false sense of security, leading people to neglect measures, such as hand hygiene and physical distancing. I cannot say this clearly enough: Masks alone will not protect you from Covid-19.”

But the changes finally bring the WHO in line with many countries around the world that have made masks mandatory in crowded public spaces, including Cuba, France, Cameroon, Vietnam, Slovakia, and Honduras. While it has not made masks a requirement, the US Centers for Disease Control and Prevention (CDC) has since April 3 suggested “wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain.”

Many health experts have wondered why it’s taken this long for the WHO to update its mask guidelines, given the accumulation of evidence that they may be helpful and have few downsides.

Eric Topol, a research methods expert and director of the Scripps Research Translational Institute, calls WHO’s delay “preposterous.” He adds, “I have great respect for the World Health Organization — but they got the mask story all wrong, and we have lost people because of it.” Lawrence Gostin, director of the O’Neill Institute for National and Global Health Law at Georgetown University, agrees, saying, “Everyone should be wearing a mask.”

Here’s what the research suggests and why experts think WHO has now revised its guidelines.

Why wear a mask?

The WHO didn’t cite any particular research for its dramatic change, noting only that it “developed this guidance through a careful review of all available evidence and extensive consultation with international experts and civil society groups.”

But there have been a number of recent studies that experts point to as the best evidence for mask use in the general public to reduce Covid-19 transmission. And a growing number of doctors, scientists, and public health experts have been calling for universal masking in indoor public spaces and crowded outdoor spaces.

One meta-review published in Lancet waded through 172 studies on Covid-19, SARS, and MERS, from 16 countries and six continents. Its authors determined that masks — as well as physical distancing and eye protection — helped protect against Covid-19.

The studies reviewed evidence both in health care and non-health care settings and then adjusted the data so they could be directly compared. The researchers found that your risk of infection when wearing a mask was 14 percent less than if you weren’t wearing a mask, although N95 masks “might be associated with a larger reduction in risk” than surgical or cloth masks.

Other literature reviews have not been as favorable. Paul Hunter, professor in medicine at the University of East Anglia, coauthored one such preprint review in early April. “In evidence-based medicine, randomized-controlled trials are supposed to trump observational studies,” he says, “And randomized-controlled trials have all been pretty much negative on face masks in the community.” The Lancet piece, he notes, gives more consideration to observational studies with surgical masks.

A few recent observational studies on mask use by the public in this pandemic, however, support general mask usage to prevent the spread of Covid-19. One from Hong Kong concluded, “mass masking in the community is one of the key measures that controls transmission during the outbreak in Hong Kong and China.” Another concluded that if 80 percent of a population were to wear masks, the number of Covid-19 infections would drop by one-twelfth, or about 8.3 percent, based on observations from several Asian countries where mask-wearing is common.

There’s been some debate over the efficacy of homemade cloth masks and surgical masks (especially compared to N95 masks, which have more evidence behind them) for the general public. But one recent article, published in the Annals of Internal Medicine, found that even cloth masks block some viral particles from escaping.

The general consensus is that masks are better at keeping your viral particles from spreading to others than keeping someone else’s from spreading to you. Catherine Clase, the lead author of the Annals of Internal Medicine piece and an associate professor of medicine at McMaster University, says one study she reviewed found even a single layer of cotton tea towel tested against a virus aerosol reduced transmission of the virus by 72 percent. “One thing to remember,” she says, “is that a mask doesn’t need to be perfect” to bring down the average number of people being infected by one sick person. “It just has to reduce the probability of transmission to some degree.”

William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center, notes that previous data on masks and viruses came out of the SARS and MERS epidemics, which involved viruses that weren’t as transmissible. “Masks were thought of then as more personal protection as opposed to community protection,” he said, helping explain why masks weren’t widely regarded as particularly effective.

But with Covid-19, the rate of asymptomatic patients may be as high as 40 percent, requiring a shift in thinking about masks from protecting the wearer to protecting the community. Clase concludes that while cloth masks may not protect you from inhaling someone else’s germs, “the evidence that they reduce contamination [from sick people] of air and surfaces is convincing, and should suffice to inform policy decisions on their use in this pandemic.”

Clase adds, “The pandemic is not going particularly well. So this is probably worth employing now and doing the additional research later.”

Why the WHO may have had trouble reaching consensus on universal masking

The WHO generally does rigorous reviews of evidence, as the whole world’s health rides on their recommendations. This may explain their delay in recommending the general public wear masks.

The agency used to largely base its decisions around expert advice, says Hunter. “They would get together a group, and they would use these experts to drive WHO guidelines.” But in 2007, a Lancet paper criticized the agency for not following evidence-based medicine, which prioritizes randomized controlled trials.

As a result, Hunter says, “WHO went through a major upheaval in its guideline development practices. Now, it has to base its recommendations on systematic reviews,” and its guideline development committees now have methodologists.

“I think [the delay] reflects a general principle often followed by scientists, which is not to change practice until the evidence is strong and definitive,” Trish Greenhalgh, a professor of primary care sciences at the University of Oxford, wrote in an email interview. “Whilst many people (including me) believe that is already the case, some scientists on WHO committees have been waiting for additional evidence to strengthen the case.”

Greenhalgh argued in early April that it was time to apply the precautionary principle to pandemic response and that the public should wear masks “on the grounds that we have little to lose and potentially something to gain.”

But David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine and a member of WHO’s Strategic and Technical Advisory Group for Infectious Hazards (STAG-IH) advisory board, says the agency “is very cautious to only use evidence when we have it. We don’t make any precautionary measures if we don’t have any contributing evidence.”

STAG-IH was asked to look into the evidence for and against mask use in early May and compiled a report for the WHO that was made public on May 25. The finding “supports mask use by the general public in the community to decrease the risk of infection,” the WHO said in a statement to Vox, noting that in updating their guidance, they took the STAG-IH advice into consideration.

Cliff Lane, the clinical director at the National Institute of Allergy and Infectious Diseases at the National Institutes of Health and another member of STAG-IH, says the WHO is ”very good at trying to get a diverse set of opinions before making recommendations.” But he admits he doesn’t know why the WHO has timed its recommendations for masks the way it has.

He is one of many experts Vox interviewed who said it’s difficult to conduct a randomized, double-blind controlled study of mask use in the general public. Because of ethics and practicality, “much of the epidemiologic data on the impact is inferred,” he says. This magnifies a general problem he sees: “Any guideline you make does an assessment of risk and benefit, and you want to get as much information as you can.” For example, if wearing a mask provides a sense of false security and encourages people to stop social distancing, then consequences may not be worth it. “It’s not a trivial decision,” he says.

Heymann says the WHO’s delay in recommendation comes in part from needing to consider evidence from around the world. “WHO takes longer because there’s a need to obtain consensus from global experts and inform six regional offices.”

Hunter added that nation-states can make decisions based in part on politics or educated guesses. “But WHO cannot take political decisions like that. It has to try to get consensus opinion among scientists, because people look to WHO to make decisions on hard evidence wherever possible.”

As Heymann sees it, “WHO is just the gold standard. Countries many times are ahead of WHO — there’s no need for them to wait for WHO to make recommendations.”

Topol, on the other hand, says the best reason he can think of for the WHO not recommending everyone wear masks is because of the worry over a global shortage of masks, particularly in the US. Perhaps, he says, “They didn’t want to have masks maldistributed, because of the dire need for, and lack of, PPE for health care personnel.”

But, he adds, “That’s not the reason to say you don’t need masks — that’s the reason to say we desperately need to make masks.”

“The world needs the WHO — and it needs it now more than ever”

The WHO has been under a lot of scrutiny since the beginning of the pandemic. And it recently got worse: At the end of May, President Trump announced that the US would pull out of the WHO altogether, potentially withdrawing a significant portion of the agency’s funding.

But the WHO isn’t alone in being slow to suggest mask use. Countries like Venezuela made masks mandatory on March 14, and the Czech Republic made the move on March 18. But the US CDC also originally recommended against masks for the public, only changing its guidance to universal masking on April 3.

Richard Besser, president of the Robert Wood Johnson Foundation and former acting director of the CDC, explains that during an emergency, experts have to look at new information and evaluating decisions. He led the emergency planning and response at the CDC for four years, and says, “When guidance went up, it was always interim. Early on, what you don’t know always exceeds what you do know, and as you learn more, you make changes.”

Sometimes those changes are minor, and sometimes, as in the case of the CDC’s mask guidance, they are significant. “In order for that to make sense to the public, you need to have something that we’re lacking right now: direct communication,” Besser says. “That’s valuable because it engenders trust in settings of crisis, where there are things people should do to protect their health. They’re much more likely to do them if they trust the messenger.”

Unlike the CDC, which has been roundly criticized for its lack of press briefings, the WHO is still holding daily conference calls during the pandemic.

“The WHO, like the CDC, is far from perfect, and is flawed in many ways,” says Gostin, the Georgetown public health law expert. “Having worked with WHO for 30 years, I can say they can be maddeningly bureaucratic and unresponsive. But the world needs the WHO — and it needs it now more than ever.”

Lois Parshley is a freelance investigative journalist and the 2019-2020 Snedden Chair of Journalism at the University of Alaska Fairbanks. Follow her Covid-19 reporting on Twitter @loisparshley.