SYNOPSIS 07/14/2020 Review of "It is time to address airborne transmission of - COVID-19"
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SYNOPSIS
07/14/2020
Review of “It is time to address airborne transmission of
COVID-19”
Article citation: Morawska L, Milton DK. It is time to address airborne transmission of COVID-19. Clin
Infect Dis. 2020 Jul 6 [Epub ahead of print]. Available from: https://doi.org/10.1093/cid/ciaa939
One-Minute Summary
The authors appeal to relevant national and international organizations and the medical
community to recognize the risk of airborne transmission of Coronavirus Disease 2019 (COVID-
19).
Evidence cited to support the potential for airborne spread of COVID-19 include the following:
Viruses are released in microdroplets during breathing, talking and coughing; the small
size of these microdroplets allow them to be suspended in air and are inhalable at
distances beyond 2 metres from their source (Morawska et al.,1 Yan et al.,2 Xie et al.,3
Lindsley et al.4).
A retrospective study (Yu et al.5) analyzed the spatial patterns of a community outbreak
of SARS-CoV-1 in Hong Kong and the authors concluded that airborne transmission may
explain the pattern of cases.
The authors cite an outbreak investigation from a poorly ventilated restaurant as
supporting evidence for aerosol transmission (Li et al.6).
The authors cite experimental data that viable “airborne virus can be exhaled” from
respiratory syncytial virus, Middle East Respiratory Syndrome coronavirus, and
influenza. The commentary states that “there is every reason to believe SARS-CoV-2
behaves similarly” to these viruses (van Doremalen et al.,7 Liu et al.8).
The authors are advocating for recognition by the World Health Organization (WHO)
and other organizations to acknowledge the potential for airborne transmission which
they believe will encourage public health interventions that include: (1) providing
sufficient and effective ventilation; (2) supplement general ventilation with local
exhaust, high efficiency air filtration, and germicidal ultraviolet lights; and (3) avoid
overcrowding.
Additional Information
None
Review of “It is time to address airborne transmission of COVID-19”PHO Reviewer's Comments
The authors use the terms “aerosol transmission” and “airborne transmission” interchangeably
although the focus of the evidence cited and supplementary measures would more accurately
be defined as aerosol transmission.
Respiratory virus transmission, including COVID-19, occurs on a spectrum from large droplets in
close contact to smaller droplets (or aerosols) that have the potential to transmit across further
distances. Referring to a pathogen as “airborne”, in a hospital setting, refers to the predominant
mode of transmission and the infection control measures necessary to protect patients and
healthcare workers from exposure. Airborne transmission occurs when airborne particles
remain suspended in the air, allowing them to travel on air currents and then inhaled by others
in the vicinity or at further distances away from the source. By this definition, COVID-19 is not an
airborne infection (such as measles or tuberculosis). In media interviews9 the authors of this
commentary agreed that hospital policies, which include droplet/contact precautions for routine
care, have been appropriate. There is broad agreement among the scientific community that the
predominant mode of COVID-19 transmission is by the droplet route10 within close contact. The
debate brought forward by this commentary is in regards to the extent that small aerosols can
be infectious.
There is a great deal of epidemiological evidence supporting droplet transmission, which this
commentary did not reference. This evidence is summarized in more detail here11 and here.12
Some notable issues with the authors’ citations include:
In the shopping mall outbreak investigation, Cai et al.13 suggest aerosolization of COVID-
19 as a possibility but air sampling was not conducted. In addition, the authors
acknowledge that fomite transmission may have occurred.
In the aerodynamic analysis at two hospitals, Liu et al.8 did not look for viable COVID-19.
In addition, the risk of infection transmission is uncertain as the quantity of viral genetic
material recovered was low and the length of time the aerosols remained afloat in the
air was not explored (see synopsis14 of the Liu et al article).
While the experiment by van Doremalen et al.7 indicates that aerosol transmission of
COVID-19 is plausible, it does not demonstrate or measure the risk of aerosol
transmission, which is dependent on an interplay of multiple factors such as droplet
size, viability, COVID-19 virulence and degree of viral shedding (see synopsis15 of the van
Doremalen et al article).
In a preprint case study, Miller et al.16 proposed that aerosol transmission was most
likely the dominant route of transmission in a superspreading event in a choir. While
supporting the idea that singing itself might have a role in aerosol transmission, Hamner
et al.17 pointed out in a peer-reviewed report of the same outbreak that there were
multiple opportunities for droplet and fomite transmission as choir members shared
snacks and were in close proximity for an extended period of time.
This commentary has generated public discussion and controversy. However, much of the
controversy can be attributed to different use of terminology and how these terms impact
hospital infection control practice and public health policy. The premise for this commentary’s
assertion of “airborne” transmission is theoretical evidence of transmission and superspreading
events possibly related to small aerosol generation, which are more likely the exception, rather
than the norm. The recent viewpoint by Klompas et al.12 in JAMA (July 13, 2020) states that long-
range aerosol-based transmission is not the dominant mode of SARS-CoV-2 transmission.
Review of “It is time to address airborne transmission of COVID-19” 2 This commentary does not advocate for N95 respirator use for routine care of COVID-19
patients. It does advocate for improved ventilation and avoidance of overcrowding. These
recommendations are not particularly controversial as evidence suggests an increased risk of
infection in crowded, poorly ventilated spaces. The extent that inhalation of small aerosols can
cause clinical infection is an area of active investigation.
The WHO,10 Public Health Agency of Canada,18 and Public Health Ontario19 continue to
recommend droplet/contact infection control precautions in healthcare settings and the use of
N95 respirators for aerosol-generating medical procedures.
References
1. Morawska L, Johnson GR, Ristovski ZD, Hargreaves M, Mengersen K, Corbett S, et al. Size
distribution and sites of origin of droplets expelled from the human respiratory tract during
expiratory activities. J Aerosol Sci. 2009;40(3):256-269. Available from:
https://doi.org/10.1016/j.jaerosci.2008.11.002
2. Yan J, Grantham M, Pantelic J, Bueno de Mesquita PJ, Albert B, Liu F, et al. Infectious virus in
exhaled breath of symptomatic seasonal influenza cases from a college community. Proc Natl
Acad Sci U S A. 2018;115(5):1081-1086. Available from:
https://doi.org/10.1073/pnas.1716561115
3. Xie X, Li Y, Chwang AT, Ho PL, Seto WH. How far droplets can move in indoor environments--
revisiting the Wells evaporation-falling curve. Indoor Air. 2007;17(3):211-25. Available from:
https://doi.org/10.1111/j.1600-0668.2007.00469.x
4. Lindsley WG, Noti JD, Blachere FM, Thewlis RE, Martin SB, Othumpangat S, et al. Viable influenza
A virus in airborne particles from human coughs. J Occup Environ Hyg. 2015;12(2):107-13.
Available from: https://doi.org/10.1080/15459624.2014.973113
5. Yu IT, Li Y, Wong TW, Tam W, Chan AT, Lee JH, et al. Evidence of airborne transmission of the
severe acute respiratory syndrome virus. N Engl J Med. 2004;350(17):1731-9. Available from:
https://doi.org/10.1056/NEJMoa032867
6. Li Y, Qian H, Hang J, Chen X, Hong L, Liang P, et al. Evidence for probable aerosol transmission of
SARS-CoV-2 in a poorly ventilated restaurant. medRxiv [Preprint – not peer reviewed]. 2020 Apr
22 [cited 2020 Jul 17]. Available from:
http://medrxiv.org/content/early/2020/04/22/2020.04.16.20067728.abstract
7. van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al.
Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med.
2020;382(16):1564-1567. Available from: https://doi.org/10.1056/NEJMc2004973
8. Liu Y, Ning Z, Chen Y, Guo M, Liu Y, Gali NK, et al. Aerodynamic analysis of SARS-CoV-2 in two
Wuhan hospitals. Nature. 2020;582(7813):557-560. Available from:
https://doi.org/10.1038/s41586-020-2271-3
9. Weeks C. Scientists studying COVID-19 raise concerns over airborne transmission. Globe and
Mail [Internet]. 2020 Jul 9 [cited 2020 Jul 17]. Available from:
https://www.theglobeandmail.com/canada/article-scientists-studying-covid-19-raise-concerns-
over-airborne-transmission/
Review of “It is time to address airborne transmission of COVID-19” 310. World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention
precautions: scientific brief [Internet]. Geneva, Switzerland: World Health Organization; 2020 Jul
9 [cited 2020 Jul 17]. Available from: https://www.who.int/publications/i/item/modes-of-
transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations
11. Ontario Agency for Health Protection and Promotion (Public Health Ontario). COVID-19 – What
we know so far about … routes of transmission. Toronto, ON: Queens’s Printer for Ontario;
2020. Available from: https://www.publichealthontario.ca/-/media/documents/ncov/wwksf-
routes-transmission-mar-06-2020.pdf?la=en
12. Klompas M, Baker MA, Rhee C. Airborne transmission of SARS-CoV-2: theoretical considerations
and available evidence. JAMA. 2020 Jul 13 [Epub ahead of print]. Available from:
https://doi.org/10.1001/jama.2020.12458
13. Cai J, Sun W, Huang J, Gamber M, Wu J, He G. Indirect virus transmission in cluster of COVID-19
cases, Wenzhou, China, 2020. Emerg Infect Dis. 2020;26(6):1343-1345. Available from:
https://doi.org/10.3201/eid2606.200412
14. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Review of
“Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals”. Toronto, ON: Queen’s Printer
for Ontario; 2020. Available from: https://www.publichealthontario.ca/-
/media/documents/ncov/research/research-liu-nature-aerodynamic-analysis-sars-cov-2-wuhan-
hospitals.pdf?la=en
15. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Review of “Aerosol
and surface stability of SARS-CoV-2 as compared with SARS-CoV-1”. Toronto, ON: Queen’s
Printer for Ontario; 2020. Available from: https://www.publichealthontario.ca/-
/media/documents/ncov/research/research-nejm-aerosol-surface-stability.pdf?la=en
16. Miller SL, Nazaroff WW, Jimenez JL, Boerstra A, Buonanno G, Dancer SJ, et al. Transmission of
SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading
event. medRxiv [Preprint – not peer-reviewed]. 2020 Jun 18 [cited 2020 Jul 17]. Available from:
http://medrxiv.org/content/early/2020/06/18/2020.06.15.20132027.abstract
17. 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(19):606-610. Available from:
https://doi.org/10.15585/mmwr.mm6919e6
18. Public Health Agency of Canada; National Advisory Committee on Infection Prevention and
Control (NAC-IPC); Salvadori M, Chung S. Infection prevention and control for COVID-19: second
interim guidance for acute healthcare settings [Internet]. Ottawa, ON: Government of Canada;
2020 [cited 2020 May 26]. Available from: https://www.canada.ca/en/public-
health/services/diseases/2019-novel-coronavirus-infection/health-professionals/infection-
prevention-control-covid-19-second-interim-guidance.htm
19. Ontario Agency for Health Protection and Promotion (Public Health Ontario). Technical brief:
IPAC recommendations for use of personal protective equipment for care of individuals with
suspect or confirmed COVID‑19 [Internet]. Toronto, ON: Queen's Printer for Ontario; 2020 [cited
Review of “It is time to address airborne transmission of COVID-19” 42020 Jul 17]. Available from: https://www.publichealthontario.ca/-
/media/documents/ncov/updated-ipac-measures-covid-19.pdf?la=en
Citation
Ontario Agency for Health Protection and Promotion (Public Health Ontario). Review of “It is time to
address airborne transmission of COVID-19”. Toronto, ON: Queen’s Printer for Ontario; 2020.
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