The World Health Organization has defined a new set of criteria for infectious diseases spread through the air following significant controversy and confusion during the pandemic about how the coronavirus was spread.
Nearly 500 experts, including epidemiologists, physicists, engineers, and public health professionals, contributed to a report published Thursday that provides a concrete breakdown of how pathogens traditionally considered “airborne” can be transmitted.
“It’s a pretty important statement of agreement to work together,” WHO’s chief scientist Jeremy Farrar told the New York Times.
Previous guidance from the WHO asserted that only a handful of pathogens with particle sizes small enough to spread across long distances could be considered airborne. The new guidance, however, suggests that other criteria, such as the speed of expulsion and concentration of pathogen particles, ought to also be considered when taking precautions during a disease outbreak.
The report explains that the descriptive phrase “through the air” is the best general term to characterize a virus where the main transmission route involves the pathogen being suspended in the air.
After respiratory pathogens are expelled from an infected person, such as by coughing or talking, they can infect others either through particles directly making contact in a “short-range, semi-ballistic trajectory” with another person or by floating in the air and entering the respiratory tract of another person.
According to the report, pathogens spread through the air can also generally be spread by contaminated surfaces, although this is less common.
Scientists and stakeholders in public health have contentiously debated for decades what characteristics of a virus make it transmittable via air, causing some confusion during other viral outbreaks, such as the first SARS outbreak in 2002 and the Ebola epidemic in 2014.
The WHO report notes, however, that the debate over “airborne” disease and “aerosol transmission” during the early stages of the COVID-19 pandemic “may have contributed to misleading information and confusion about how pathogens are transmitted in human populations.”
Despite debates over masking during the COVID-19 pandemic, the WHO report does not provide clear guidelines for under what conditions wearing a mask could prevent the spread of pathogens through the air.
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The coalition agreed, however, on “the importance of adequate ventilation and airflow patterns within indoor spaces” to lower transmission risk, but acknowledged that “prevention and mitigation measures need to be tailored differently for different pathogens and settings.”
Under the prior definition of airborne diseases, hospital systems developed guidelines to ensure patient and worker safety, including isolation rooms and protective gear, including N95 respirators, to avoid inhaling fine droplet particles. The new WHO guidance does not provide clear advice for hospitals on how the new definitions should be applied.