Exploring how the COVID-19 pandemic highlighted gaps in research about the transmission of respiratory diseases: Q&A with Dr. Nicole Bouvier

February 16, 2022

The COVID-19 pandemic has made research into virus transmission a top scientific priority. SARS-CoV-2, the virus that causes COVID-19, can be spread through virus-infected saliva droplets and particles. The details of how far and fast saliva droplets and particles disperse through the air, and under what circumstances, remain under investigation.

Dr. Nicole Bouvier, Associate Professor in Infectious Diseases and Microbiology at the Icahn School of Medicine at Mount Sinai, discusses how the COVID-19 pandemic has highlighted gaps in old evidence about how infectious diseases are transmitted between humans leading up to the American Association for the Advancement of Science's (AAAS) panel, 'Transmission of Airborne Pathogens through Expiratory Activities' happening at the AAAS Annual Meeting on February. 18, 2022. Learn more about the panel here.

What does your current research focus on?

I study the transmission of influenza viruses using animal models to simulate human-to-human virus spread under laboratory conditions.  I am most interested in how well animal models model us -- the ways in which they faithfully represent what happens in human influenza, and the ways in which they don't, so that we can best utilize animal models to inform individual and public health.  That overarching goal takes many forms, including the immunology and physiology of influenza in animals, as well as the physical and environmental parameters that govern efficient virus transmission in humans and animals.

Has the COVID-19 pandemic highlighted gaps in old evidence about how infectious diseases are transmitted between humans?

Absolutely.  There have been several longstanding misunderstandings about how respiratory diseases transmit among humans.  The one that most impacted public health guidance early in the pandemic was the idea that, if a respiratory disease spreads most efficiently when infected and susceptible hosts are in "close contact," then that means that the mode of transmission is likely to be either by contact or large respiratory droplets.  In other words, that the virus is transferred between people either by touch (i.e., infected person coughs into his hand then shakes the hand of a susceptible person, who then rubs her own nose, or an infected person sneezes on a computer keyboard that is later used by the susceptible person, who then rubs her nose with virus-contaminated fingers) or by ballistic "droplet sprays" of respiratory particles from an infected person's cough or sneeze, which impact directly upon the respiratory mucosa of a susceptible person in close proximity.  Very few respiratory diseases were considered "airborne" transmissible, meaning that the pathogen is carried in exhaled respiratory particles -- called aerosols -- that are light enough and small enough to remain suspended in the air for minutes to hours and to be transported far from the infected person on air currents before being inhaled by a susceptible person.  

Because early data suggested that SARS-CoV-2 is most efficiently transmitted by people in “close contact,” public health agencies erroneously inferred that the virus was transmitting via contact or droplet sprays.  However, they (and generations of public health experts before them) were confusing transmission mechanism with transmission probability:  Pathogen transmission in aerosols is simply more likely to occur over short interhost distances than long ones, mainly due to dilutional effects.  Aerosol particles can just as easily be inhaled near to or far from the emitting host; however, the likelihood of inhalation is greatest when the pathogens in those aerosol particles are maximally concentrated within the air, i.e., nearest to the emitter (just as smoke is densest nearest to a smokestack and gets more and more dilute as the smoke particles float farther and farther away).  Because of the relative rarity of long-range transmission events that can be definitively attributed to pathogens in aerosol particles, it was thought that the airborne route was of minimal importance in most respiratory diseases, but the converse isn't true: you cannot rule out transmission by aerosol particles just because transmission occurs most often at short interhost distances.  Frequent transmission events between people in "close contact" could just as easily be happening by "short-range" aerosol particles as by contact or droplets.  

Can you share a small taste of what people might expect to learn from you on the AAAS panel, 'Transmission of Airborne Pathogens through Expiratory Activities'?

It's hard to say - I guess it depends on what questions people ask!  In my video seminar (which accompanies the panel discussion), I talk about the above misconception in reference to another virus, respiratory syncytial virus (RSV), in which experimental data were similarly misinterpreted to classify RSV as a "contact" transmissible virus.  (I believe that the videos are only going to be accessible to conference attendees, though?)

Another misconception that my co-panelist Sima Asadi and I have done a lot of research on is the past tendency to overemphasize the importance of explosive respiratory emissions, like coughs and sneezes, in the transmission of respiratory pathogens. We hypothesized that speaking was likely to release large numbers of respiratory particles too, just spread over a longer time frame -- but no one seemed to be regarding talking as a potential generator of infectious, airborne respiratory viruses.  (Think about the CDC's "respiratory etiquette" campaign -- "cover your cough or sneeze."  No one says, "cover your speech"!)  Sima very elegantly showed that speech does indeed release comparable numbers of respiratory particles over time as a cough, and that the louder you talk, the more respiratory particles you emit.  That may at least partially explain why many COVID outbreaks have occurred in the setting of public speaking or singing (churches and choir practices) or in places where people tend to raise their voices to be heard over ambient noise (bars and restaurants).

Learn more about AAAS and click here to register to attend the 2022 AAAS Annual Meeting.

Written by Kelsea Franzke


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We acknowledge that UBC’s campuses are situated within the traditional territories of the Musqueam, Squamish and Tsleil-Waututh, and in the traditional, ancestral, unceded territory of the Syilx Okanagan Nation and their peoples.


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