Transmission Dynamics of COVID-19

Contributed by Dr Rini Bandyopadhyay

Respiratory pathogens may be transmitted by the airborne route or by the droplet route. The mode of respiratory transmission of SARS-CoV-2 is not completely understood. The primary mode of transmission is through respiratory droplets which spread through person-to-person contact and through indirect routes, such as contaminated surfaces. The possibility of opportunistic aerosol spread, wherein special circumstances such as aerosol generating procedures (AGP) within hospitals facilitate airborne transmission of a normally droplet-transmitted disease, has been proposed. The mere presence of airborne particles is not evidence of true airborne transmission of SARS-CoV- 2 virus. Establishing airborne transmission requires data to supporting that it remains infective over time or distance. In a WHO analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported.

In a study by van Doremalen et al, experimentally generated aerosol particles with SARS-CoV-2 were found to have virus that was viable in cell culture throughout the 3 hours of aerosol testing, with marked exponential decay in virus titre. Such laboratory-generated aerosols may not be exactly analogous to human exhaled aerosols. A similar study from the University of Nebraska found viral RNA in nearly two-thirds of air samples collected in isolation rooms in a hospital treating people with severe COVID-19 and in a quarantine facility housing those with mild infections; however, further testing did not identify any viable, infectious virus in these samples. In China, aerosol particles were isolated from air samples collected from various areas in 2 hospitals and from outdoor spaces in Wuhan; virus genome was detectable in some aerosols but at very low concentrations.

A case report from Singapore among 35 healthcare workers wearing surgical masks while in close contact with a COVID-19 positive patient during an aerosol-generating procedure, showed no test positivity for SARS-CoV-2 two weeks after exposure. A recently published MMWR report investigated high rates of secondary transmission of COVID-19 among attendees of a choir practice in Washington, raising concerns of airborne spread of COVID-19. However, the investigators concluded that the close and prolonged contact of attendees was highly conducive to disease spread via droplets and fomites.

These findings reinforce the fact: 1) that detection of viral RNA in environmental samples based on PCR-based assays is not indicative of viable virus, and, 2) the presence of SARS-CoV-2 RNA in air samples does not necessarily imply that it could be effectively transmissible via the airborne route. Lastly, there is some evidence that COVID-19 infection may lead to intestinal infection and be present in faeces. However, to date only one study has cultured the COVID-19 virus from a single stool specimen. There have been no reports of faecal−oral transmission of the COVID- 19 virus to date.

Selected References

  1. Li Q, Guan X, Wu P, et al. Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia. N Engl J Med 2020;382(13):1199–207
  2. World Health Organization. Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24 February 2020
  3. van Doremalen N et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. 2020;382:1564-1567
  4. Ong SW 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 2020 Mar 4;323(16):1610-1612