According to Nature, the World Health Organization (WHO) has changed its classification of pathogens transmitted through the air, redefining how viruses and other pathogens spread through the air.
This redefinition, in the works for two years, aims to provide clear information for future pandemics. However, some researchers remain skeptical of the new definition, leading to ongoing debates.
Virologists now acknowledge that the novel coronavirus primarily spreads through aerosols, tiny particles that can linger in the air for hours, in addition to transmission through larger droplets containing virus particles on surfaces like hands, or through short-range respiratory droplets.
During the COVID-19 pandemic, the WHO did not promptly clarify that the virus spreads through the air, only publicly acknowledging the importance of airborne transmission in October 2020. Soumya Swaminathan, the WHO's Chief Scientist at the time, stated in November 2022 that based on existing evidence, this acknowledgment should have come earlier. However, it wasn't until December 2021 that the WHO revised its official documents on COVID-19 transmission, recognizing that the virus can spread "pathogenically" through the air, even as a primary route. Therefore, early COVID-19 prevention recommendations focused mainly on surface cleaning, handwashing, and social distancing, rather than mask-wearing and improving ventilation.
Some researchers suggest that if the WHO had acknowledged airborne transmission of the virus earlier, more lives could have been saved.
To clarify the distinctions between airborne transmission, droplet transmission, and aerosol transmission for better pandemic preparedness, the WHO engaged over 100 experts from various scientific disciplines in over two years of deliberation, culminating in the release of a technical report on April 18 (referred to as the report).
Traditionally, particles larger than 5 micrometers are considered droplets, while those smaller than 5 micrometers are aerosols. However, the report no longer differentiates between "droplets" and "aerosols," introducing the concept of "respiratory infectious particles" to describe these particles, regardless of size.
Nevertheless, the report avoids categorizing all airborne pathogens as airborne transmission, instead using the inclusive term "through the air" to describe the spread of any pathogen "shuttling through or suspended in the air." Furthermore, the report further divides this into two categories: "airborne transmission/inhalation," where respiratory infectious particles are inhaled from the air, and "direct deposition," where particles land on a person's mouth, nose, or eyes within a short distance.
According to this classification, the novel coronavirus is considered an airborne transmission/inhalation pathogen, with a much lower risk of direct deposition transmission.
For many scientists, this classification has its pros and cons.
"One positive aspect I see in this report is the elimination of the 5-micrometer division between aerosols and droplets," said Lidia Morawska, an aerosol scientist at the Queensland University of Technology in Australia. However, he believes the report's differentiation between airborne transmission and direct deposition is unreasonable and confuses the issue.
Linsey Marr, an environmental engineer at Virginia Tech who contributed to the report, believes that the term "airborne transmission" is more concise and logical than the description "through the air" in the report.
Julian Tang, a clinical virologist at the University of Leicester in the UK who participated in the report, stated that scientists have engaged in intense debates over the term "airborne transmission" and its implications. Some scientists believe that using "airborne transmission" for all pathogens transmitted through the air is problematic. "They think it's too scary, too loaded, and could cause panic, so they chose the description 'through the air,'" Tang said.
Nevertheless, Tang noted that the report sets an important benchmark for the world to address future pandemics.
"The next epidemic is likely to be caused by respiratory viruses, as they are usually the fastest mutating pathogens," Tang said. The report's clarity on transmission modes will help public health departments worldwide make appropriate responses. They will consider early mask-wearing, ventilation, and other preventive measures, as there is now a precedent.
