In the early days of the Covid pandemic, a team of scientists called to the World Health Organization to acknowledge that the disease could be spread through the air.
Initially, the agency rejected them, despite growing evidence that coronavirus-laden droplets stuck in the air, making indoor spaces hotbeds of infection. The researchers responded with a public campaign, which helped convince the World Health Organization to finally acknowledge, in late 2021, that Covid was airborne.
In the wake of the controversy, the agency also asked a panel of advisers — including some of its scientific critics — to update its official guidelines for classifying the ways pathogens are spread. After more than two years of discussion, this group published one report defining new definitions that could have significant implications for countries around the world that depend on the agency to set policies to limit the spread of the disease.
The WHO’s previous position was that only a few pathogens – those that travel in small droplets and spread over long distances, such as tuberculosis – could be considered airborne. But the new report suggests broader categories that aren’t based on droplet size or distance spread. Such changes were controversial because they raised the prospect that more diseases may now require costly control measures, such as hospital isolation rooms and protective equipment.
“It’s an important first step,” said Dr. Ed Nardell, a tuberculosis specialist at Harvard Medical School and a member of the team. “We really have a beginning, with agreed terminology, even if not everyone is happy with it.”
Before the pandemic, the WHO and other agencies typically recognized some modes of disease transmission. One was by “contact transmission,” in which someone picked up a pathogen either by directly touching an infected person or through contact with a contaminated surface.
“Droplet transmission” involved the short-range spread of disease when people coughed or sneezed droplets larger than 5 microns (five millionths of a metre), which then landed directly in the victim’s mouth, eyes or nose.
“Airborne transmission” referred to only a few diseases that spread in droplets smaller than 5 microns, floating long distances until inhaled.
When Covid emerged, the agency said it was likely to spread over short distances, either through contact or droplet transmission.
But Yuguo Li, a mechanical engineer at the University of Hong Kong, and many other critics worried that the WHO was ignoring the possibility that Covid could spread through the air. As the pandemic progressed, scientists found evidence in outbreaks that the coronavirus can indeed spread long distances in floating droplets. (Some scientists respondent the power of these studies.)
The WHO formed the new advisory group in November 2021 and asked Dr Li to co-chair. At the group’s meetings, Dr Lee and others argued that the agency was based on false dichotomies.
For example, there is little scientific basis for the 5 micron limit for small droplets. Larger droplets can also remain on the surface for long periods of time.
The researchers also argued that short-range infections were not proof that a disease is spread only through coughing and sneezing. Infected people can also exhale droplets through breathing or talking, which are then inhaled by other people nearby.
The new report divides transmission routes into those involving contact and others involving air. The team agreed to call the second route “through the air”.
Linsey Marr, an environmental engineer at Virginia Tech and a member of the advisory group, found this phrase more curious than a simpler term like airborne transmission.
“I find it very clumsy,” he said. “But we were looking for the lowest common denominator terminology that everyone could live with.”
The report further clarified that pathogens can spread through the air in two ways. One is “direct deposition,” which refers to droplets hitting the mucous membranes of the mouth, eyes, or nose. The other is “airborne/inhalation”, in which droplets are inhaled.
After scientists came up with the new terminology, the WHO received agreements from the US Centers for Disease Control and Prevention, as well as its counterparts in Africa, China and Europe, to adopt the same definitions.
“It’s a pretty significant statement of agreement to work together,” said Dr Jeremy Farrar, the WHO’s chief scientist.
However, the new report made no recommendations on how agencies should stop the spread of disease through these different routes. The authors acknowledged that they were unable to reach a consensus on this issue.
Traditionally, hospital guidelines for controlling airborne diseases required expensive measures, such as isolation rooms with negative air pressure, as well as N95 respirators and other protective equipment to prevent inhalation of fine droplets. But it is not clear which diseases warrant such screening or which efforts should be made outside hospitals.
Dr. Walter Zingg, an infectious disease expert at the University of Zurich and a member of the advisory group, said the old categories offered clearer guidance. Staying a few meters away from someone who is coughing and sneezing was once thought to be an effective way to avoid spreading droplets, for example.
“It was simplistic and probably not true in some ways, but it served a purpose,” he said. “Now we have to bring other variables to the table.”
Dr. Farrar said such guidelines should be based on clear experimental evidence, although such evidence is still scarce for many diseases. Scientists still debate, for example, the extent to which influenza, which has been studied for more than a century, spreads through the air.
“We know a certain amount, but we’re not absolutely sure,” said Dr. Farrar. “This is the kind of work we desperately need on influenza.”