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Based on previous experience with SARS, MERS-CoV, and other human coronaviruses (hCoVs), a number of early assumptions were made regarding the transmission of SARS-CoV-2. The virus, we were told, was almost certainly spread primarily by close contact (1 meter) with a symptomatic case, and was transmitted by large droplets, and possibly contaminated fomites.
Asymptomatic (or presymptomatic) spread, or longer distance transmission via aerosols, was considered unlikely. And so public health policies (mask recommendations, social distancing, etc.) were based on these assumptions.
Over time, our understanding of the SARS-CoV-2 virus, and the disease processes it causes, have evolved. Asymptomatic spread - once dismissed as either rare, or insignificant - is now viewed as an important driver of the pandemic (see EID Journal: Asymptomatic or Presymptomatic Transmission Of SARS-CoV-2).
Gradually, aerosols have been given more weight in transmission of the virus, as well - although they are generally still characterized as capable of traveling only short distances (1 or 2 meters), and remaining infectious and aloft for a short period of time (minutes).
Some recent studies include:
MMWR Early Release: COVID-19 Superspreading Event In A Church Choir
J. Infect. Dis.: Airborne or Droplet Precautions For COVID-19?
COVID-19: The Airborne Division
Is the coronavirus airborne? Experts can’t agree - Nature
Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (April 1, 2020) - Nat. Academy Sci.
The change in policy recommendations for wearing face covers has come, in part, due to the realization that some aerosolized spread of the virus likely occurs simply from breathing and speaking, and that could be reduced by a cloth barrier (see COVID 19 can spread through breathing, talking, study estimates).
Questions over the persistence, and viability of bioaerosols remains a matter of debate; How long does aerosolized SARS-CoV-2 remain airborne? How far can it travel? And how long it remain infectious?
Attempting to answer some of these questions, we have a dispatch - published in the CDC's EID Journal - that describes research performed at four aerobiology laboratories (Tulane University, NIH-IRF , Fort Detrick, MD,; USAMRIID, Fort Detrick, MD, USA; and University of Pittsburgh) across the country.
While parts of the dispatch are highly technical, the short version is they found that SARS-CoV-2 was better suited to spread via bioaerosols, and took longer to decay than either SARS or MERS-CoV.
In fact, rather than measuring its viability in an aerosol in minutes, SARS-CoV-2 showed the ability to remain infectious for hours under specific lab conditions (temp, humidity, no UV, etc.). How long that would translate in real-world conditions is unknown, but it would likely be longer than either SARS or MERS-CoV.
This study suggests that SARS-CoV-2 may not only be viable as an airborne pathogen, but that infected persons may produce bioaerosols that remain infectious for long periods of time.
Both findings have potential implications for future environmental guidance on reducing the spread of the virus in large enclosed areas (see Monday's ECDC Technical Report: Heating, ventilation and air-conditioning systems in the context of COVID-19).
While this isn't settled science, it brings us closer to understanding the transmissibility of the pandemic virus. I've only posted some excerpts, so follow the link to read it in its entirety.
Volume 26, Number 9—September 2020
Dispatch
Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions
Alyssa C. Fears, William B. Klimstra, Paul Duprex, Amy Hartman, Scott C. Weaver, Kenneth S. Plante, Divya Mirchandani, Jessica Ann Plante, Patricia V. Aguilar, Diana Fernández, Aysegul Nalca, Aysegul Totura, David Dyer, Brian Kearney, Matthew Lackemeyer, J. Kyle Bohannon, Reed Johnson, Robert F. Garry, Doug S. Reed1, and Chad J. Roy1
Suggested citation for this article
Abstract
We aerosolized severe acute respiratory syndrome coronavirus 2 and determined that its dynamic aerosol efficiency surpassed those of severe acute respiratory syndrome coronavirus and Middle East respiratory syndrome. Although we performed experiment only once across several laboratories, our findings suggest retained infectivity and virion integrity for up to 16 hours in respirable-sized aerosols.
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a readily transmissible zoonotic pathogen and the etiologic agent of the coronavirus disease (COVID-19) pandemic (1). To determine aerosol stability of the virus, we measured the dynamic (short-term) aerosol efficiencies of SARS-CoV-2 and compared its efficiency with SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV).
(SNIP METHODS)
Conclusions
The comparison of short-term aerosol efficiencies of 3 coronaviruses showed SARS-CoV-2 approximates or exceeds the efficiency estimates of SARS-CoV and MERS-CoV. Some efficiency determinations for SARS-CoV-2 ranged to −5.5log10 (Figure 1), a full log difference from MERS-CoV. The higher efficiencies across independent laboratories strengthens this observation. These data suggest that SARS-CoV-2 generally maintains infectivity at a respirable particle size over short distances, in contrast to either betacoronavirus.
Aerosol suspension results suggest that SARS-CoV-2 persists longer than would be expected when generated as this size particle (2-µm mass median aerodynamic diameter). This finding is notable because decay and loss in the infectious fraction of airborne virus would be expected on the basis of prior susceptibility studies with other environmentally hardy viruses, such as monkeypox virus (5). A recent study (6) showing only a slight reduction of infectivity in aerosol suspensions with approximately similar particle sizes also suggested minimal effects on SARS-CoV-2 airborne degradation.
Collectively, these preliminary data suggest that SARS-CoV-2 is resilient in aerosol form and agree with conclusions reached in earlier studies of aerosol fitness (6). A clear limitation of the aerosol stability data is that we report only 1 measurement of the 16-h time point; future studies need to repeat these findings before any definitive conclusions are reached. Aerosol transmission of SARS-CoV-2 may be a more important exposure transmission pathway than previously considered (7).
Our approach of quantitative measurement of infectivity of viral airborne efficiency augmented by assessment of virion morphology suggests that SARS-CoV-2 may be viable as an airborne pathogen.
Humans produce aerosols continuously through normal respiration (8). Aerosol production increases during respiratory illnesses (9,10) and during louder-than-normal oration (11). A fraction of naturally generated aerosols falls within the size distribution used in our experimental studies (<5 μm), which leads us to conclude that SARS-CoV-2–infected persons may produce viral bioaerosols that remain infectious for long periods after production through human shedding and airborne transport. Accordingly, our study results provide a preliminary basis for broader recognition of the unique aerobiology of SARS-CoV-2, which might lead to tractable solutions and prevention interventions.
Ms. Fears is a doctoral candidate in the Biomedical Sciences Program, Department of Microbiology and Immunology, Tulane School of Medicine, Tulane University. Her primary research interests include infectious disease and immunopathogenesis of respiratory viral pathogens.
