The World Health Organization has issued a report that transforms how the world understands respiratory infections like covid-19, influenza, and measles.
Motivated by grave missteps in the pandemic, the WHO convened about 50 experts in virology, epidemiology, aerosol science, and bioengineering, among other specialties, who spent two years poring through the evidence on how airborne viruses and bacteria spread.
However, the WHO report stops short of prescribing actions that governments, hospitals, and the public should take in response. It remains to be seen how the Centers for Disease Control and Prevention will act on this information in its own guidance for infection control in health care settings.
The WHO concluded that airborne transmission occurs as sick people exhale pathogens that remain suspended in the air, contained in tiny particles of saliva and mucus that are inhaled by others.
While it may seem obvious, and some researchers have pushed for this acknowledgment for more than a decade, an alternative dogma persisted — which kept health authorities from saying that covid was airborne for many months into the pandemic.
Specifically, they relied on a traditional notion that respiratory viruses spread mainly through droplets spewed out of an infected person’s nose or mouth. These droplets infect others by landing directly in their mouth, nose, or eyes — or they get carried into these orifices on droplet-contaminated fingers. Although these routes of transmission still happen, particularly among young children, experts have concluded that many respiratory infections spread as people simply breathe in virus-laden air.
“This is a complete U-turn,” said Julian Tang, a clinical virologist at the University of Leicester in the United Kingdom, who advised the WHO on the report. He also helped the agency create an online tool to assess the risk of airborne transmission indoors.
Peg Seminario, an occupational health and safety specialist in Bethesda, Maryland, welcomed the shift after years of resistance from health authorities. “The dogma that droplets are a major mode of transmission is the ‘flat Earth’ position now,” she said. “Hurray! We are finally recognizing that the world is round.”
The change puts fresh emphasis on the need to improve ventilation indoors and stockpile quality face masks before the next airborne disease explodes. Far from a remote possibility, measles is on the rise this year and the H5N1 bird flu is spreading among cattle in several states. Scientists worry that as the H5N1 virus spends more time in mammals, it could evolve to more easily infect people and spread among them through the air.
Traditional beliefs on droplet transmission help explain why the WHO and the CDC focused so acutely on hand-washing and surface-cleaning at the beginning of the pandemic. Such advice overwhelmed recommendations for N95 masks that filter out most virus-laden particles suspended in the air. Employers denied many health care workers access to N95s, insisting that only those routinely working within feet of covid patients needed them. More than 3,600 health care workers died in the first year of the pandemic, many due to a lack of protection.
However, a committee advising the CDC appears poised to brush aside the updated science when it comes to its pending guidance on health care facilities.
Lisa Brosseau, an aerosol expert and a consultant at the Center for Infectious Disease Research and Policy in Minnesota, warns of a repeat of 2020 if that happens.
“The rubber hits the road when you make decisions on how to protect people,” Brosseau said. “Aerosol scientists may see this report as a big win because they think everything will now follow from the science. But that’s not how this works and there are still major barriers.”
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