What is the evidence for droplet transmission for SARS-CoV-2?

A guest post from Dr Evonne Curran

The disputed airborne mode of transmission in this pandemic requires further scrutiny. Researchers have thus far focused on presenting a case for airborne transmission1 rather than disputing that the ‘primary’ mode of transmission for SARS-CoV-2 is via droplets2.

In July 2020, the World Health Organization (WHO) stated that SARS-CoV-2 was “primarily spread through droplets and close contact”.2   Of note, ‘close contact’ merely acknowledges that an infected person is near to a susceptible one when transmission occurred. It defines the station of departure and arrival of the pathogen – but not the route taken. The WHO elaborated in saying “droplet transmission can occur when a person is in close contact (within 1 meter) of an infected person who has respiratory symptoms (e.g. coughing or sneezing) or who is talking or singing. Transmission of SARS-CoV-2 can occur through direct, indirect or close contact with infected secretions such as saliva and respiratory secretions or their respiratory droplets, which are expelled when an infected person coughs, sneezes talks or sings”.2 The WHO used 16 references to support the droplet transmission statements – all were examined.

Only 5 of the 16 papers2, specify a respiratory route of transmission, i.e., droplet or airborne. The remainder detail the places of ‘close contact’, e.g., ‘household transmission’, without further elaboration. The first is a WHO3 investigation on the pandemic itself which states: “COVID-19 is transmitted via droplets and fomites during close unprotected contact between an infector and infectee.”3 There are no references to support this claim. The WHO further stated: “Airborne spread has not been reported for COVID-19 and it is not believed to be a major driver of transmission based on available evidence3.” Neither the ‘available evidence’ nor definitions as to how such an assessment was, or could be, made were provided. There is a statement that aerosol transmission could be “envisaged if certain aerosol-generating procedures are conducted’.3 Although this statement lacks assurance, nosocomial airborne transmission is not an identified ‘knowledge gap’ although community airborne transmission is. That staff in China wear N95 masks was detailed, but not discussed.3

The second paper describes an outbreak following singing practice4. The authors are not definitive on which transmission mode occurred when stating “…the act of singing itself might have contributed to transmission through emis­sion of aerosols” and “choir practice attendees had multiple opportunities for droplet transmission from close contact”.4   Aerosol and droplet transmission definitions are omitted.

The third and fourth papers also describe outbreaks5-6. The authors of a restaurant outbreak, state the “most likely route of transmission is droplet” 5 but they also state that transmission cannot be explained by droplet transmission alone.5 They suggest that strong airflow from an air conditioner could have propagated droplets via the air in directions different from those expelled from the index case. They also state that transmission was beyond the 1m distance accepted for droplet transmission5. Ergo, although the authors conclude droplet transmission was the most likely route, they describe a mode of transmission that meets the airborne definition. The fourth is an assessment of pre-symptomatic clusters of transmission6 where the authors suggest a transmission route but omit evidence, i.e., “pre-symptomatic transmission might occur through generation of respiratory droplets or possibly through indirect transmission.”6

The final is a report of outbreaks in fitness classes7 which states “the moist, warm atmosphere in a sports facility coupled with turbulent air flow generated by intense physical exercise can cause more dense transmission of isolated droplets.”7 The authors commented on the closeness of instructors to pupils without specifying distances. They assume droplet transmission but fail to describe air-flows or air quality.”7 There is no mention as to why (or whether) airborne transmission was either considered or excluded.

Thus, although the WHO’s 16 citations2 describe transmission whilst people are in the same buildings, family settings etc., the evidence presented does not merit the descriptor of being ‘primarily’ transmitted via droplets. The previously held paradigm that close proximity means droplet transmission have been dispelled7. Closeness to a case neither defines droplet, nor excludes airborne or other modes of transmission. WHO2 presented evidence of closeness to cases but not specific transmission routes. Others have presented more compelling evidence for airborne transmission.1

Airborne precautions require both good ventilation and effective respiratory protection, e.g., FFP3 masks. Whilst nosocomial transmission exists, urgency is required to identify if there is a failure of process (masks and ventilation being insufficiently effective) or a failure to follow process (people failing or unable to follow guidance). Even at this late stage, a precautionary approach of recommending ventilation and safe respiratory protection is needed.

In conclusion, the WHO evidence that SARS-CoV-2 is primarily transmitted via droplets is unconvincing. Unless and until the WHO provides definitive evidence that SARS-CoV-2 is primarily spread via the droplet route, all statements to that effect should be amended or removed.

References:

1            Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D. and Schooley R. Ten scientific reasons in support of airborne transmission of SARS-CoV-“. Lancet.  Published Online April 15, 2021.  doi.org/10.1016/ S0140-6736(21)00869-2

2            World Health Organisation, (WHO) Transmission of SARS-CoV-2: implications for infection prevention precautions. Scientific Brief 9th July 2020. Transmission of SARS-CoV-2: implications for infection prevention precautions (who.int)   Last accessed 28/11/2021

3            Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24 February 2020. Geneva: World Health Organization; 2020 ( https://www.who.int/docs/default-source/coronaviruse/who-china-joint-mission-on-covid-19-final-report.pdf). Last accessed 28/11/2021

4            Hamner L, Dubbel P, Capron I, Ross A, Jordan A, Lee J, et al. High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice — Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020;69:606-10.

5            Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. Early Release-COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerg Infect Dis. 2020;26(7):1628-1631.

6            Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 – Singapore, January 23-March 16, 2020. MMWR Morb Mortal Wkly Rep. 2020;69(14):411-5.

7            Jang S, Han SH, Rhee J-Y. Cluster of Coronavirus Disease Associated with Fitness Dance Classes, South Korea. Emerg Infect Dis. 2020;26(8).

8            Tang JW, Marr LC, and Milton DK. Aerosols should not be defined by distance travelled. Journal of Hospital Infection 115 (2021) 131e132c.

2 thoughts on “What is the evidence for droplet transmission for SARS-CoV-2?

  1. Thanks Evonne – you have done something I have been trying to find time to do for ages – but can only be done by looking at all the papers and the data – and see what each one REALLY says. Part of the problem is to understand what each person means when they use the word “airborne”. Even for droplets there must be some element of travel via the air?

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