COVID-19: SARS-CoV-2 Aerosol Mechanisms

Airborne Viral Transformation

Viral denaturing

While large droplets generated in the upper respiratory tract and oral cavity sediment rapidly and travel only a short distance (typically <2 m), the smaller aerosol particles (typically < 5μm diameter) generated in the lower respiratory tract and larynx can remain suspended and airborne for many minutes or hours.1,2 It has been suggested that any particles <100μm in diameter should be considered as showing similar aerodynamic behaviour, remaining airborne over distances longer than 2m.3 During airborne transport, pathogens may lose viability and infectivity, and their survival decay dynamics must be understood.4,5

Higher level of certainty:

  1. Although some pathogens are only transmitted by the airborne aerosol route, referred to as obligate airborne transmission (e.g. Tuberculosis), some may transmit preferentially (e.g. measles and smallpox), and others may transmit opportunistically by the airborne route, transmitting by other routes but with airborne transmission possible under certain situations.6 Other coronaviruses (e.g. SARS-CoV-1 and MERS) are considered to be opportunistically spread by the airborne route.6,7
  2. While suspended in small respirable aerosol particles, the airborne survival of pathogens (including SARS-CoV-2) is dependent on the environmental conditions, specifically relative humidity (RH) and temperature. At intermediate RHs and room temperature, the survival decay times for SARS-CoV-1 and SARS-CoV-2 are similar, with half-lives of 1-2 hours in tissue culture media or artificial saliva.8,9,10
  3. Viral RNA has been identified in air samples in a number of studies: RNA genome copy numbers per litre of sampled air have spanned a large range from undetectable or close to the limit of detection (0.001-0.042 copies/L)11 to >3 copies/L of air.12,13 By comparison, air samples have reported up to 10,000 copies / L of air for influenza virus.14
  4. Both steady equilibrium solute concentrations in aerosol particles at a particular RH can lead to steady osmotic stresses on pathogens, rates of change of solute concentrations may also be influential in determining pathogen survival.15,16,17
  5. Exposure of airborne SARS-CoV-2 to light (particularly UV-A, 315-400 nm, and UV-B light, 280-315 nm) has shown that survival is significantly reduced, with decay half-times reduced to under 10 minutes in the strongest lighting conditions.8
  6. Early air sample measurements were unable to identify infectious SARS-CoV-2 virus although other human respiratory viruses were isolated and identified in a COVID clinic.13,18 One study has identified infectious SARS-CoV-2.19 Low virus concentrations in airborne samples and the challenges of ensuring the integrity of viruses are not compromised during sampling are challenges in identifying infectious virus.

Lower level of certainty:

  1. The dependence of airborne survival on relative humidity and aerosol particle composition remains uncertain. In one study, SARS-CoV-2 survived for longer at intermediate RH than high RH in tissue culture medium, with the reverse trend in artificial saliva.10
  2. Although the airborne transmission of SARS-CoV-2 has been established between ferrets20 and the airborne transmission in humans has been inferred (e.g. during a choral society rehearsal),21 the relationship between RNA genome copy numbers and infectious virus remains uncertain.
  3. The complex interactions of multiple pathogens, viral load and respiratory droplet composition and how this depends on the infection state of the individual remain to be addressed.

References:

  1. Johnson, G. R. et al. Modality of human expired aerosol size distributions. J. Aerosol Sci. 42, 839–851 (2011).
  2. Nicas, M., Nazaroff, W. W. & Hubbard, A. Toward understanding the risk of secondary airborne infection: Emission of respirable pathogens. J. Occup. Environ. Hyg. 2, 143–154 (2005).
  3. Drossinos, Y. & Stilianakis, N. I. What aerosol physics tells us about airborne pathogen transmission. Aerosol Sci. Technol. 0, 1–5 (2020).
  4. Asadi, S., Bouvier, N., Wexler, A. S. & Ristenpart, W. D. The coronavirus pandemic and aerosols : Does COVID-19 transmit via expiratory particles ? Aerosol Sci. Technol. 54, 635–638 (2020).
  5. Roy, C. J. & Milton, D. K. Airborne Transmission of Communicable Infection – The Elusive Pathway. N. Engl. J. Med. 350, 1710–1712 (2004).
  6. Tellier, R., Li, Y., Cowling, B. J. & Tang, J. W. Recognition of aerosol transmission of infectious agents: A commentary. BMC Infect. Dis. 19, 1–9 (2019).
  7. Schuit, M. et al. Airborne SARS-CoV-2 is Rapidly Inactivated by Simulated Sunlight. J. Infect. Dis. 222, 564–571 (2020).
  8. van Doremalen, N. et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N. Engl. J. Med. 382, 1564–1567 (2020).
  9. Smither, S. J., Eastaugh, L. S., Findlay, J. S. & Lever, M. S. Experimental Aerosol Survival of SARS-CoV-2 in Artificial Saliva and Tissue Culture Media at Medium and High Humidity. Emerg. Microbes Infect. 9, 1415–1417 (2020).
  10. Lin, K. & Marr, L. C. Humidity-Dependent Decay of Viruses, but Not Bacteria, in Aerosols and Droplets Follows Disinfection Kinetics. Environ. Sci. Technol. 54, 1024–1032 (2020).
  11. Marr, L. C., Tang, J. W., Van Mullekom, J. & Lakdawala, S. S. Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. J. R. Soc. Interface 16, 20180298 (2019).
  12. Yang, W., Elankumaran, S. & Marr, L. C. Relationship between Humidity and Influenza A Viability in Droplets and Implications for Influenza’s Seasonality. PLoS One 7, 1–8 (2012).
  13. Liu, Y. et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 582, 557–560 (2020).
  14. Guo, Z.-D. et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020. Emerg. Infect. Dis. 26, (2020).
  15. Chia, P. Y. et al. Detection of air and surface contamination by SARS-CoV-2 in hospital rooms of infected patients. Nat. Commun. 11, 2800 (2020).
  16. Thompson, K. A. et al. Influenza Aerosols in UK Hospitals during the H1N1 (2009) Pandemic – The Risk of Aerosol Generation during Medical Procedures. PLoS One 8, (2013).
  17. Richard, M. et al. SARS-CoV-2 is transmitted via contact and via the air between ferrets. Nat. Commun. 11, 1–6 (2020).
  18. Miller, S. L. et al. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event. MedRxiv doi:10.1101/2020.06.15.20132027
  19. Lednicky, J. A. et al. Collection of SARS-CoV-2 Virus from the Air of a Clinic within a University Student Health Care Center and Analyses of the Viral Genomic Sequence. Aerosol Air Qual. Res. 20, 1167–1171 (2020).

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