Saturday, March 28, 2020

COVID-19: The journey of a viral pandemic

Modes of transmission of SARS-CoV-2

A novel human coronavirus that emerged in Wuhan, China in the later months of 2019 has now dissipated all around the world, causing a pandemic. Let's analyze how this virus manages to spread so virulently breaching our usual barriers.

Modes of transmission of SARS-CoV-2

Modes of potential spread:

a) Respiratory droplets
b) Fomite transmission
c) Airborne transmission
d) Intestinal infection via feces

Respiratory Droplet Transmission:

  • Particle size: >5-10 microns
  • The radius of exposure: <1m  from a COVID-19 positive case
  • The case must have respiratory symptoms (cough, sneeze, shortness of breath)
  • The contact exposes the droplets to his/her mucosae: mouth, nose, and eyes to droplets 

Fomites in the immediate environment:

  • Direct contact with a COVID-19 positive case
  • Indirect contact: immediate environment or objects coming in contact with the case (stethoscope, thermometer, case files, etc)

Airborne transmission:

  • Particle size: <5 microns
  • The radius of exposure > 1 meter from a COVID - 19 positive case
  • Particles are droplet nuclei that remain in the air for longer periods of time 

Intestinal infection via feco-oral route:

SARS-CoV-2 has been cultured from stool specimens. But there have been no reports of feco-oral transmission of COVID-19 virus. [2][3] Recent studies have suggested potential feco-oral transmission. [4]


A recent publication in the New England Journal of Medicine, reports from settings where symptomatic COVID-19 patients have been admitted in which no COVID-19 RNA was detected in air samples. [5]

They studied the stability of the SARS-CoV-2 virus in 5 experimental conditions:
  • Aerosol (viability of the virus for 3 hours)
  • Plastic (up to 72 hours)
  • Stainless steel (up to 48 hours)
  • Copper
  • Cardboard 

They found that the stability of this virus on plastic and steel was far more than on copper and cardboard.

Their results indicated that aerosol and fomite transmission of SARS-CoV-2 is plausible since the virus can remain viable and infectious in aerosols for hours and on surfaces up to days (depending on the inoculum shed). These findings echo those with SARS-CoV-1, in which these forms of transmission were associated with the nosocomial spread and super-spreading events,5 and they provide information for pandemic mitigation efforts.

In addition, it is important to note that the detection of RNA in environmental samples based on PCR based assays are not indicative of a viable virus that could be transmissible.

Another study done by The Journal of the American Medical Association shows that the air samples were negative despite the extent of environmental contamination. [6] It also demonstrates that swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents.

"The strength of a chain is determined by its weakest link.
Make sure you're not the weakest link"

Stay safe and hang in there, we'll make it through,
Real soon :)

Written by Anagha Rao

[1] World Health Organization. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24 February 2020 [Internet]. Geneva: World Health Organization; 2020 Available from:
[2] Zhang Y, Chen C, Zhu S et al. [Isolation of 2019-nCoV from a stool specimen of a laboratory-confirmed case of the coronavirus disease 2019 (COVID-19)]. China CDC Weekly. 2020;2(8):123–4. (In Chinese)
[3] Tang A, Tong Z-d, Wang H-l, Dai Y-x, Li K-f, Liu J-n, et al. Detection of novel coronavirus by RT-PCR in stool specimen from asymptomatic child, China. Emerg Infect Dis. 2020 Jun.
[4] Gu J, Han B, Wang J, COVID-19: Gastrointestinal manifestations and potential fecal-oral transmission, Gastroenterology (2020), doi:
[5] van Doremalen, N., Bushmaker, T., Morris, D. H., Holbrook, M. G., Gamble, A., Williamson, B. N., et al. (2020). Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. https://doi.org10.1056/NEJMc2004973.
[6] Ong SWX, Tan YK, Chia PY, et al. Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient. JAMA. Published online March 04, 2020. doi:10.1001/jama.2020.3227

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