COVID-19 Can Have Airborne Transmission but You Don't Need to Run for an N95
There is virtually no doubt that SARS-CoV2 is transmitted by droplets and contact. However, the debate continues about whether SARS-CoV2 can be transmitted through the air, in what epidemiologists call “airborne transmission.” As with most biologic processes, unfortunately this is not a dichotomy. Many (too many) factors play a role.
Population density matters. As people breathe, speak, sneeze, or cough we all produce many particles that have a continuum of sizes. These particles are unfortunately called too many names in the literature and the lay press (e.g., droplets, aerosols). Viruses and biologic processes don’t read textbooks. These particles can be large (what healthcare epidemiologists call “droplets”), medium size (no fancy name for them), and small (these are called “aerosols” by some but “droplet nuclei” by others). If we are near only one infectious person, the number of small particles (aerosols) expelled may not be enough to meaningfully contribute to infection. But if we are exposed to many infectious people at once, the number of small particles can increase. In such instances, airborne transmission in addition to contact and droplet transmission can play a role in outbreaks.
Patient characteristics are also tremendously important. Some may extrapolate that COVID is not as contagious or rule out the possibility of airborne transmission because of a paucity of hospital outbreaks, even if not following airborne precautions. If we follow the natural history of COVID, we now know that a person is possibly infectious 48 hours before symptom onset. Most people do not require hospitalization, and those that require hospitalization may be in later stages of the disease. We are learning daily that COVID, the disease caused by SARS-CoV2, is likely a continuum. Initially, the disease is predominantly caused by direct injury of the virus to tissues, but as days go by some patients will have immunologic or para-infectious syndromes that may require hospitalization. By the time a patient with COVID requires hospitalization, their infectiousness has likely decreased. It is now clearly recognized that presence of viral RNA does not equal risk of transmission in many cases.
The setting is also very important. How big is the space where the infectious person and their potential contact are located. If outdoors, the risk is tremendously decreased as air flows freely greatly decreasing the possibility of breathing “the same air.” Indoors, the number of air exchanges is very important: the more air exchanges, the lesser the likelihood of spread. Fortunately, most hospitals have already implemented an increased number of air exchanges likely decreasing the possibility of airborne transmission of pathogens in hospitals.
If airborne transmission plays a role in SARS-CoV-2 transmission, I believe it is predominantly in the early stages of the disease, in the viral phase. That may explain why most healthcare outbreaks have occurred in nursing homes and long-term care facilities. Not only because of potential infection prevention deficits but because patients are already in the facility when they become infectious. They are at the peak of infectiousness when in the facility. Hospitals on the other hand, will usually admit patients days or even weeks after the beginning of the infectious period, likely attenuating the risk of transmission in hospitals.
Recognizing that SARS-coV2 can also spread via small particles should not lead to panic. It should lead us to modify our behaviors in the community by avoiding crowded indoor settings, using universal source control with face coverings, and maintaining physical distance.
|Modified from CDC.|
Reducing population density in healthcare facilities (patient census and personnel) can lead to increased safety but has a tremendous impact on population health (less capacity to take care of patients) and potential economic implications if healthcare personnel numbers are decreased. Engineering controls are also costly but fortunately most hospital design standards already address increased air exchanges compared to regular buildings and homes.
This pandemic has been challenging for all. COVID-19 keeps me humble as what I thought I knew yesterday may not be true today. Let’s all remain humble and nimble as we respond to COVID-19 in the community and in healthcare facilities.