Q&A: WHO updates definition of airborne transmission
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Key takeaways:
- WHO updated its definition of what constitutes an airborne pathogen.
- Under the new definition, pathogens including COVID-19 and measles are now considered airborne.
WHO recently updated its definition of airborne transmission, expanding the list of pathogens that can be spread through the air.
Responding to yearslong debate during the COVID-19 pandemic about what constitutes airborne transmission — and whether SARS-CoV-2 should be considered airborne — a group of experts brought together by WHO created the umbrella term “transmission through the air,” which describes two modes of spread:
- “airborne transmission/inhalation,” when an infectious respiratory particle is inhaled; and
- “direct deposition,” when expelled particles land on a person’s mouth, nose or eyes.
The purpose of the new definition was to minimize confusion around how to describe the transmission of pathogens through the air that can potentially cause infection in humans, which has generally varied across scientific disciplines and organizations. Under the new definition, the size of the droplet does not matter anymore.
We spoke with Mark Reeves, PhD, professor of physics at George Washington University, about what the new definition means and what pathogens are now considered airborne.
Healio: Can you explain the new definition of airborne transmission?
Reeves: The way I see it, it basically asks the question, “How does the pathogen enter the human body?” If it enters in through the respiratory system and is transmitted out of somebody else's respiratory transmission system, then it's considered to be airborne transmission. They've tried to simplify it to get away from a lot of jargon, but basically, if it leaves another person's mouth or nose, goes into the air and then is brought up into a second person through the mouth or the nose by breathing in the air, then it's airborne transmission.
Healio: What are some of the pathogens that would now be considered airborne that were not under the previous definition of airborne transmission?
Reeves: I think there's a lot of literature that shows evidence for airborne transmission diseases like measles, and the cold and flu.
As far as a public health approach, though, that transmission mode wasn't impressed upon as much as “contact transmission.” In Western countries, the approach to mitigating diseases is by washing hands and cleaning surfaces, which assumes that the transmission is done by contact — you somehow get it on your hands, it goes from your hands to another person's hands, and then they touch their nose or their eyes or their mouth and then they get the disease.
Honestly, I don't think we thought much about airborne transmission as a general public, but in East Asian countries, they did think about it. Even before COVID-19, there was a very large part of the population that would wear masks during the cold and flu season, and I think the reason for that is their exposure to SARS and MERS 10 or 20 years ago. It conditioned them to look at airborne transmission as a possible way of catching those diseases.
Healio: Are there many experts who would argue that this new definition is wrong? If so, what would be the basis of their argument?
Reeves: A person might want to make the definition overly complicated by saying, for example, that the particles have to be below a certain size or above a certain size. It's known that the trajectory for larger particles is more falling to the floor and other surfaces, whereas the very small particles stay airborne for up to hours. However, that becomes a distracting discussion because the dynamics of particle size is rather complicated.
Once a large particle is expelled through breathing or speaking forcefully, it can actually shrink in size through evaporation and then become a small particle, but if it lands on a surface or on someone's hands, or if someone sneezes in their hands, and then shake hands with somebody, those are all not considered airborne transmission.
The simple definition — going from one nose to another nose, or one mouth to another mouth through the air — is one that is hard to argue with. Any argument would be getting into how it gets from point A to point B, but what really matters is that it does get from point A to point B through the air.
Healio: What was at the heart of the disagreement about the potential airborne transmission of SARS-CoV-2 early in the pandemic?
Reeves: People, particularly in the West, in the U.S. and in Europe, weren't prepared to think about airborne transmission, which is a two-step process: first, realize that it's a possibility; and second, once you realize that, what do you do about it?
In my research group, we're developing a technology that allows one to map air currents in indoor spaces that are complex. We'll have people moving through the spaces to see how that changes the air currents, and we're able to then map where airborne particles would go, where they would concentrate and how they would disperse.
The problem we run into then is people ask the question, “If you map my space in my building, what do I do if you find a problem?” That's a tough one. There's a moral obligation that's incurred. I think that people become defensive and think that they've created a hazard.
Those are hard questions that go beyond the definition of airborne transmission.
It wasn't until a group of leading aerosol scientists wrote a letter in Nature that asked the authorities to really think about airborne transmission that there was some shift in the thinking — and even then, it took some very crucial experiments. There was evidence of things like choir singing and transmission on cruise ships, but then there was a group that did experiments on golden hamsters because they can transmit and develop the same symptoms as we do from COVID-19. They put the hamsters in cages and did a test where they sprayed SARSs-CoV-2-containing solutions in their food, sprayed the solution right on the nose of the hamsters, and put hamsters next to each other with an air tube connecting their cages.
What they found was that the ones who got the virus via their food got sick, whereas the ones who got it sprayed right up their nose got really sick, really fast. The hamsters sleeping next to the ones who had COVID-19 got just as sick just as quickly as the ones who had been sprayed. So, this was a pretty convincing experiment that showed airborne transmission was a very lethal pathway.
Those sorts of experiments develop a growing awareness, but even today, it's really a public health question. How do you get people to change their behavior to mitigate a certain transmission pathway for a disease?
Healio: How would that conversation be different today with this new definition?
Reeves: I think the new definition is sensible and it provides a very believable and unambiguous way to identify airborne transmission. People are conditioned by now to accept that that is a legitimate way to get a disease, and so it will make it easier to first identify a transmission path that was controversial — in the case of COVID — and begin to put measures in place in an emergency situation.
References:
Global technical consultation report on proposed terminology for pathogens that transmit through the air. https://www.who.int/publications/m/item/global-technical-consultation-report-on-proposed-terminology-for-pathogens-that-transmit-through-the-air. Published April 18, 2024. Accessed April 29, 2024.
Handley A, et al. Viruses. 2023;doi:10.3390/v15030748.
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