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COVID-19 Virus Remains Infectious Up to 16 Hours in Air
  Mark Mascolini
SARS-CoV-2, the coronavirus that causes COVID-19, remained infectious for up to 16 hours when suspended in air as an aerosol, according to results of research at US labs [1]. Short-term airborne fitness of SARS-CoV-2 (ability to copy itself) proved similar to or greater than fitness of the coronaviruses that cause SARS (SARS-CoV) and Middle Eastern respiratory syndrome (MERS-CoV).
These findings appear in medRxiv, which presents research not yet peer-reviewed or published.
Rapid global spread of SARS-CoV-2 indicates the high transmissibility of this novel coronavirus. The virus can travel in droplets and remain viable on surrounding surfaces where it lands [2]. When virus-containing droplets dehydrate, SARS-CoV-2 lingers in aerosols that stay suspended in air [3]. Aerosols can be emitted when an infected person talks or just breathes [1]. Aerosol production increases during respiratory illness [4].
Researchers from 4 US labs—Tulane University, the National Institutes of Health, the US Army Medical Institute for Infectious Diseases, and the University of Pittsburgh—conducted a new study to measure SARS-CoV-2 aerosol efficiency (fitness) and persistence under controlled conditions. They used three different nebulizers to produce viral aerosols and conducted efficiency experiments in the four separate aerobiology labs.
Short-term aerosol efficiency or spray factor (Fs) divides initial viral titer (level) by the resultant aerosol to yield an indicator of airborne viral fitness. Within 1 minute of aerosol generation by the Collision 3-jet nebulizer in 3 labs, Fs (fitness) of SARS-CoV-2 was slightly but significantly greater than Fs of SARS-CoV (P = 0.0254) or MERS-CoV (P = 0.0153).
The Tulane lab alone calculated long-term stability of airborne SARS-CoV-2. They released aerosols into a rotating drum to create a static aerosol suspension and collected samples at 10 and 30 minutes and at 2, 4, and 16 hours after rotation began. Prevailing ambient conditions were 23 +/- 2 degrees Celsius and 53 +/- 11% relative humidity.
A minor but constant fraction of SARS-CoV-2 maintained replication competence (infectivity) up to and including the 16-hour measurement. The viral decay curve proved so flat that the researchers could not determine biological half-life of SARS-CoV-2. Aerosol concentration measured as viral genome copies declined minimally across the study period.
Scanning electron microscopy showed that SARS-CoV-2 virions suspended as aerosols for 10 minutes or 16 hours were similar in shape and general appearance to virions seen before aerosolization. That finding also suggests retained replication competence.
The researchers conclude that aerosol efficiency of SARS-CoV-2 is “on par with or exceeding the efficiency estimates of SARS-CoV and MERS-CoV.” In addition, SARS-CoV-2 maintained infectivity when airborne over short distances and for longer than one would expect with a small particle that can easily be inhaled (2 uM mass median aerodynamic diameter).
The investigators proposed that SARS-CoV-2 “is remarkably resilient in aerosol form” and that aerosol transmission of the virus “may in fact be a more important exposure transmission pathway than previously considered [5].”
1. Fears AC, Klimstra WB, Duprex P, et al. Comparative dynamic aerosol efficiencies of three emergent coronaviruses and the unusual persistence of SARS-CoV-2 in aerosol suspensions. medRxiv preprint doi: https://doi.org/10.1101/2020.04.13.20063784. (medRxiv presents research not yet peer-reviewed or accepted for publication.)
2. Guo ZD, Wang ZY, Zhang SF, et al. Aerosol and surface distribution of severe acute respiratory syndrome coronavirus 2 in hospital wards, Wuhan, China, 2020. Emerg Infect Dis. 2020;26(7). doi: 10.3201/eid2607.200885.
3. Anfinrud P, Stadnytskyi V, Bax, BA, Bax A. Visualizing speech-generated oral fluid droplets with laser light scattering. N Engl J Med. April 15, 2020. doi: 10.1056/NEJMc2007800. https://www.nejm.org/doi/full/10.1056/NEJMc2007800
4. Proano A, Bravard MA, Lopez JW, et al. Dynamics of cough frequency in adults undergoing treatment for pulmonary tuberculosis. Clin Infect Dis. 2017;64:1174-1181.
5. Pung R, Chiew CJ, Young BE, et al. Investigation of three clusters of COVID-19 in Singapore: implications for surveillance and response measures. Lancet. 2020;395:1039-1046.

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