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After Push From Experts, World Health Organization Says It’s Possible COVID-19 Spreads by Air

3 minute read

The World Health Organization (WHO) on Thursday heeded calls from more than 200 scientists, who urged the global health authority to acknowledge that COVID-19 may spread by air.

Previously, the WHO said contact with large respiratory droplets, like those expelled in a sick person’s cough or sneeze, appeared to be the primary way COVID-19 spreads. But in a highly publicized letter published earlier this week, a large group of scientists argued the WHO’s guidance neglected to adequately address another important route of transmission: inhaling tiny respiratory particles generated by a sick person, which can remain suspended in the air indoors for hours.

In a scientific brief published Thursday, the WHO allowed that “short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out.” Still, it maintained that further studies on airborne transmission are needed, and said the evidence is strongest for respiratory droplet transmission.

The WHO hinted at its evolving opinion during a press briefing on Tuesday, when Maria Van Kerkhove, the agency’s technical lead for COVID-19, said its scientists had been “talking about the possibility of airborne transmission and aerosol transmission.” Many of the scientists who signed the letter on airborne transmission celebrated her comments.

“Nice work, team Indoor Air,” signatory Joseph Allen, an indoor air expert and assistant professor at the Harvard T.H. Chan School of Public Health, tweeted on Tuesday.

In addition to making peace with the scientific community, the WHO’s recognition could change public-health officials’ disease-prevention advice, especially as more indoor spaces reopen to the public. That could mean new standards for indoor ventilation, for example, or mandates for wearing face masks inside stores, restaurants and other public facilities.

The WHO’s new scientific brief also addressed another COVID-19 controversy: the role asymptomatic people play in spreading the virus.

Van Kerkhove sparked a flurry of confusion last month, when she said it was “very rare” that people without symptoms of coronavirus infect others—a statement seemingly contrary to months of public-health messaging. Van Kerkhove later qualified her comments, emphasizing that asymptomatic spread is possible but happens with unknown frequency.

The WHO’s new brief echoes her comments. “While someone who never develops symptoms can also pass the virus to others,” it says, “it is still not clear to what extent this occurs and more research is needed in this area.”

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