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The WHO Says Airborne Coronavirus Transmission Isn’t a Big Risk. Scientists Are Pushing Back

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For months, the World Health Organization (WHO) has said COVID-19 spreads mainly via direct contact with large respiratory droplets, like those expelled in a sick person’s cough or sneeze. In a letter published this week in Clinical Infectious Diseases, 239 scientists say the agency may be wrong.

It’s only the latest chapter in an ongoing tug of war between the WHO and the rest of the public-health world. “This is one in a series of many miscues,” says Dr. Eric Topol, director and founder of the Scripps Research Translational Institute. “It’s really unfortunate how the WHO has led to all sorts of confusion.”

The new letter, which was co-written by a WHO consultant and reviewed by experts from more than 30 countries, argues the WHO and other health authorities are not paying enough attention to airborne COVID-19 transmission—that is, infection via inhaling tiny respiratory droplets that can linger in the air.

WHO officials acknowledged that possible route of transmission at a press conference Tuesday, after the letter was published, but said it continues to collect evidence. “We have been talking about the possibility of airborne transmission and aerosol transmission as one of the modes of transmission of COVID-19,” Maria Van Kerkhove, the WHO’s technical lead for COVID-19, said.

Studies of other viruses completed before the pandemic have “demonstrated beyond any reasonable doubt” that droplets expelled by sick individuals can “remain aloft in air and pose a risk of exposure at distances beyond 1 to 2 [meters] from an infected individual,” the letter says. More recent research suggests the same is true of SARS-CoV-2, the virus that causes COVID-19. In some reported cases, people have gotten sick after being in the same room as an infected individual, even if they didn’t have close or sustained contact.

Think of it like cigarette smoke, says Linsey Marr, a professor of civil and environmental engineering at Virginia Tech and one of the letter’s signatories. The cloud is most concentrated around the person smoking, but it also disperses and drifts throughout the room. Viral aerosol functions much the same way, she says.

The WHO’s COVID-19 guidance has addressed airborne spread in health care environments, since some procedures can aerosolize the virus, but stopped short of calling it a threat to the general public. A WHO spokesperson told TIME “the topic is presently being reviewed by our technical experts.”

The letter’s 239 signatories say that’s not good enough. “There’s been a lot of emphasis on hand-washing and on social distancing, but if they [the WHO] acknowledge that aerosol transmission is happening, we can have additional guidance” about things like ventilation techniques and wearing masks whenever people congregate indoors, Marr says. Even very simple guidance, like keeping doors and windows open when possible, could help, she adds.

There’s nothing to stop cities or countries from codifying these sorts of precautions on their own. But as the world’s preeminent global health authority, the WHO’s words—and silence—carry weight. If the WHO doesn’t publicly recognize risks like airborne transmission, “it just gives naysayers more fodder to deny the truth,” Topol says.

The letter is only the latest example of the roiling tension between the WHO and the wider scientific community.

The fast-moving COVID-19 pandemic has exposed the WHO’s weak spots. For one thing, the underfunded and overburdened global health agency cannot enter countries to do fieldwork without permission, and relies on its member states to provide much of the data used in its analyses.

The WHO also moves with the slow pace and risk aversion of a bureaucratic organization, even when the world is demanding new and better information about the coronavirus every day. “The evidence for aerosol transmission is there; it’s just maybe not as compelling as they would like,” Topol says. “I call it a purist view.”

That institutional caution helps explain why it took the WHO until June 5 to recommend that people in high-transmission areas wear fabric face masks in public—a recommendation that was, by then, already standard in many countries and cities. “Every recommendation that we put out needs to be applicable for every type of situation. That’s a blessing and a curse,” Van Kerkhove told TIME when the mask guidance came out.

Topol says that bar is sometimes too high. “What do you have to lose” by recommending extra precautions, he asks. “Go with the best evidence and the best expert opinion. Maybe it’s not perfect evidence. But it’s good enough.”

Despite holding press briefings almost every day, the WHO has also stumbled when communicating with the general public. In an oft-criticized tweet from January, for example, the WHO declared that, “Preliminary investigations conducted by the Chinese authorities have found no clear evidence of human-to-human transmission of the novel #coronavirus.”

To the WHO, that mean the threat of widespread human-to-human transmission—though possible—was still under investigation. To the average person, it meant human-to-human transmission wasn’t a big threat. So when it became clear that COVID-19 does pass from person to person, many people lost faith in the WHO.

For many, that feeling was compounded last month when Van Kerkhove called asymptomatic coronavirus transmission “very rare” during a press conference, despite several studies and months of expert warnings to the contrary. She later walked back the comment.

These incidents are more than communication slip-ups. Topol fears they’ll erode trust in the WHO—and in science more broadly—at the exact moment that confidence is critically important.

“WHO is a venerable institution, something that we need, that we rely on,” Topol says. “Each time one of these things happens, where there’s a serious misalignment with the truth and the science and the evidence, you wind up with another credibility titer reduction. We need to go the other direction.”

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Write to Jamie Ducharme at jamie.ducharme@time.com