Still No Definitive Evidence of Airborne Transmission of the Novel Coronavirus
The FLARE Four
- Respiratory particles, produced every time we talk, cough, sneeze, shout or sing, vary in size from fine aerosol particles and droplet nuclei (less than five microns) to larger droplets
- Airborne transmission requires pathogens in fine aerosol particles to remain infectious over distance and time
- Recent evidence suggests SARS-CoV-2, the novel coronavirus, may remain aerosolized for several hours, but the presence of the virus in aerosols or air samples is not evidence of airborne transmission
- Available evidence supports the transmission of SARS-CoV-2 primarily through droplets and contact routes
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The rapid spread of SARS-CoV-2 across the globe has prompted scientists to revisit the age-old question: Is this virus spreading via the airborne route?
Sarimer Sánchez, MD, research fellow, Erica Shenoy, MD, PhD, associate chief of the Infection Control Unit, and David Hooper, MD, chief of the Infection Control Unit and associate chief of the Division of Infectious Diseases at Massachusetts General Hospital, review the evidence in a fast literature update posted on May 31, 2020.
Droplets and Aerosols
Respiratory particles, produced every time we talk, cough, sneeze, shout or sing, vary in size:
- Droplets are defined by the CDC as particles more than five microns in size. Due to their larger size, they are believed to fall to the ground quickly. Diseases transmitted via droplets are spread through close person-to-person contact, typically at a distance less than six feet
- Fine particle aerosols and droplet nuclei are smaller than five microns. They may remain suspended in the air, and they may be inhaled into the respiratory tract, whereas larger droplets typically settle in the upper respiratory tract
The CDC defines airborne transmission as occurring when organisms in respiratory particles remain infectious over distance and time. Classic long-range airborne transmission has been established for only a very small number of pathogens, notably measles and tuberculosis.
SARS-CoV-2 in Air
Two publications have reported on the presence of SARS-CoV-2 in air samples:
- A letter published in New England Journal of Medicine reports three-hour viability of SARS-CoV-2 (although with an exponential decay in virus titer) in aerosols that were experimentally generated with a nebulizer
- A non-peer reviewed pre-print in medRxiv reports that air sampled in patient rooms and hallways contained SARS-CoV-2 viral RNA; however, there was no viable, infectious virus in these samples
Other Studies
Investigators recently reported, in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report, on the high rates of secondary transmission of SARS-CoV-2 at a choir practice in Washington state. This superspreading event, heavily featured in the media, raised early concerns about airborne transmission of SARS-CoV-2. The authors do cite the possibility of aerosol emission via speech and singing, but they conclude the close and prolonged contact of attendees was highly conducive to disease spread via droplets and contact with surfaces.
Data analyzing exposures during hospital care of patients with COVID-19 continue to support droplet and contact routes as the predominant modes of SARS-CoV-2 transmission. It is important to keep in mind that the presence of SARS-CoV-2 virus in aerosols is not evidence of airborne transmission, and that further evidence is needed to determine if this virus remains infectious over distance and time.
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