4. Modes of transmission

4.1 Droplets & aerosols
4.2 Mode of transmission
4.3 Survival time of SARS-CoV-2

4.1 Droplets & aerosols

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Droplets & Aerosols


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SARS-CoV-2 Coronavirus Micro-droplets – NHK World report

NHK World

Droplets: small liquid drops that are generated by expiratory events such as coughing, sneezing, laughing, talking or even breathing. 

  • Large droplets: usually brought down to ground under gravity and are transmitted over a limited distance (0.3-1.0 m).
  • Small droplets: water content of large droplets may evaporate in air, producing smaller droplet that could travel up to 1.5 m.
  • Droplet nuclei: water content of small droplets may evaporate in air, producing even smaller droplet nuclei that could then remain suspended in air and could be transmitted over a long distance (up to 50 m).

Aerosols: Suspensions of small liquid droplets or solid particles in air. 

Cowling BJ, Ip DKM, Fang VJ, et al. (2013): Aerosol transmission is an important mode of influenza A virus spread. Nat Commun 4: 1935-1935.

Further reading

4.1.1 Infectious Micro-droplets

  • SARS-CoV-2 Coronavirus Micro-droplets – NHK World report

4.2 Modes of transmission

(1) Droplet: Transmitted through direct and indirect means. It can be inhaled directly by others when the infected patient cough or sneeze. In contrast, droplets may settle on table surface, and later on when another person touches the table before touching his own mucosal surfaces (including nose, mouth and eyes), the transmission of disease might then happen.

(2) Direct contact: Transmitted from the infected to the healthy through direct physical contacts.

(3) Indirect contact: Transmitted through fomites.

(4) Aerosols (possible): Aerosols are capable of traveling a longer distance and longer time in air, compared to typical droplets, before settling down owing to the small size and light weight. Aerosols can be generated during aerosol-generating procedures in hospital settings, such as resuscitation, endotracheal intubation, positive pressure ventilation, tracheostomy insertion, bronchosocopy, and airway suction, etc. Coughing, sneezing, toilet flushing, cigarette smoke and hot pot steam, etc. could also generate infective aerosols, further studies are needed to  test these hypothesis.

Barker J & Jones MV (2005): The potential spread of infection caused by aerosol contamination of surfaces after flushing a domestic toilet. Journal of Applied Microbiology 99: 339-347.
Best EL, Sandoe JAT & Wilcox MH (2012): Potential for aerosolization of Clostridium difficile after flushing toilets: the role of toilet lids in reducing environmental contamination risk. Journal of Hospital Infection 80: 1-5.
Tran K, Cimon K, Severn M, Pessoa-Silva CL & Conly J (2012): Aerosol Generating Procedures and Risk of Transmission of Acute Respiratory Infections to Healthcare Workers: A Systematic Review. PLOS ONE 7: e35797.

4.3 Survival time of SARS-CoV-2

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Survival time of SARS-CoV-2


  • Viable SARS-CoV-2 could be detected in aerosol up to 3 hours after aerosolization, raising concerns on the possibility of viral transmission through aerosolization. 
  • Viable SARS-CoV-2 could only be as long as 2-3 days on plastic and stainless steel. Indirect transmission of SARS-CoV-2 through fomite is highly likely! We must always remain vigilant and be aware of our personal hygiene to reduce the risk of infection!

Van Doremalen N, Bushmaker T, Morris DH, et al. (2020): Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. New England Journal of Medicine: In press.