After hundreds of experts urged the World Health Organization to review mounting scientific research, the agency acknowledged Tuesday that airborne transmission of the coronavirus may be a threat in indoor spaces.

WHO expert committees are going over evidence on transmission of the virus and plan to release updated recommendations in a few days, agency scientists said in a news briefing.

The possibility of airborne transmission, especially in “crowded, closed, poorly ventilated settings, cannot be ruled out,” said Dr. Benedetta Allegranzi, who leads the WHO’s committee on infection prevention and control.

She said the agency recommends “appropriate and optimal ventilation” of indoor environments, as well as physical distancing.

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Agency staff fielded several questions from reporters about transmission of the virus by air, prompted by an open letter from 239 experts calling on the agency to review its guidance. Many of the letter’s signatories have collaborated with the WHO and served on its committees.

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Experts who signed the letter welcomed the WHO’s announcement.

“We are very glad that WHO has finally acknowledged the accumulating evidence, and will add aerosol transmission indoors to the likely modes of transmission” for the coronavirus, said Jose-Luis Jimenez, a professor of chemistry at the University of Colorado, Boulder. “This will allow the world to better protect themselves and fight the pandemic.”

In their letter, Jimenez and other scientists called on the WHO to recommend that people avoid overcrowding, particularly on public transportation and in other confined spaces. Public buildings, businesses, schools, hospitals and care homes should also supply clean air, minimize recirculating air, and consider adding air filters and virus-killing ultraviolet lights, they said.

“Public health agencies around the world take their cues from WHO, and hopefully this will lead to greater emphasis on wearing of face coverings and avoiding the three Cs: close contact, closed and poorly ventilated spaces, and crowds,” said Linsey Marr, an aerosol expert at Virginia Tech. “These measures will help slow the pandemic and save lives.”

WHO scientists said that for the past few months, the infection prevention committee has been weighing the evidence on all the ways in which the coronavirus spreads, including by tiny droplets or aerosols.

“We acknowledge that there is emerging evidence in this field, as in all other fields,” Allegranzi said. “And therefore, we believe that we have to be open to this evidence and understand its implications regarding the modes of transmission and also regarding the precautions that need to be taken.”

It will also be important to understand the importance of transmission by aerosols compared with larger droplets, and the dose of the virus needed for infection from aerosols, she said.

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“These are fields that are really growing and for which there is evidence emerging, but it is not definitive,” she said. “However, the evidence needs to be gathered and interpreted, and we continue to support this.”

Other experts said it has been clear for some time that airborne transmission of the virus is possible but agreed that it’s not yet certain how big a role this route plays in spreading the virus.

“The question of how important it is for overall transmission remains an open one,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

Still, he and other experts have said that the WHO is too slow and cautious in adopting precautions based on emerging evidence.

WHO scientists offered an explanation for their seemingly slow pace. On average, they review 500 new papers a day, many of which turn out to be of dubious quality, said Soumya Swaminathan, the WHO’s chief scientist.

As such, the scientists have to review the quality of each paper before including it in their analysis, she said: “Any guidance we put out has implications for billions of people around the world. It has to be carefully done.”