Mitigating airborne transmission of SARS-CoV-2

Abstract

E1010 CMAJ | JULY 5, 2021 | VOLUME 193 | ISSUE 26 © 2021 CMA Joule Inc. or its licensors A t the outset of the COVID-19 pandemic, understanding of respiratory virus transmission was founded on a conceptual framework that dichotomized infectious particles into droplets that are > 5 μm and “drop” close to the source, versus aerosols that are < 5 μm and remain suspended in air. However, aerosol scientists challenged this traditional categorization: particles as large as 100 μm can remain suspended in air and be inhaled.1 In November 2020, the Public Health Agency of Canada (PHAC) recognized that SARS-CoV-2 can be transmitted by aerosol, and more recent updates from the World Health Organization (WHO)2 and Centers for Disease Control and Prevention (CDC)3 highlight both shortand long-range aerosol transmission. Despite this, public health guidance in Canada has not been adequately updated to address airborne transmission, possibly because of ongoing debate over its relative importance compared with other modes of transmission. We argue that it is not necessary to reach consensus on the predominance of airborne transmission before making meaningful changes to Canada’s public health guidance. We call for updated messaging and policies on ventilation and masks — informed by multidisciplinary collaboration — to ensure the highest standards of safety for Canada’s communities. The CDC states that SARS-CoV-2 spreads by exposure to infectious respiratory fluids through aerosol inhalation, deposition of droplets (splashes and sprays), and touch, and that aerosol inhalation, although more likely at close range because aerosols are more concentrated at their source, also occurs at longer distances, depending on environmental conditions.3 Airborne transmission of SARS-CoV-2 is supported by its asymptomatic transmission, superspreading events, long-range transmission confirmed by contact tracing and genomic sequencing, nosocomial infections despite strict adherence to contact and droplet personal protective equipment (PPE), and animal studies showing transmission via an air duct.4 Although air sampling of live pathogens in nonlaboratory settings is challenging — researchers have been unsuccessful with measles and tuberculosis, both diseases with airborne transmission — viable SARS-CoV-2 has been sampled from the air in a hospital room and a car driven for 15 minutes by a mildly symptomatic patient.4 Tuberculosis also infects primarily those in close proximity despite having long-range transmission potential. Having established that SARS-CoV-2 can infect at both short and long range by aerosol inhalation, use of the term “airborne” accurately reflects transmission through the air and permits clear communication with the public. The importance of ventilation and filtration, wearing masks indoors even when distanced, and choosing outdoor spaces for gathering becomes more intuitive when airborne transmission is explicitly acknowledged. Ventilation is a key element in the mitigation against longrange aerosol transmission, and actionable advice and funding to prioritize ventilation assessments and upgrades should be widely disseminated to businesses and schools. Unproven mitigation measures like “deep cleaning” and plexiglass installations as barriers to droplet spread persist as misplaced investments that can provide a false sense of security. The risk of fomite transmission is estimated to be low and plexiglass can actually impede air flow, potentially leading to increased aerosol transmission.5 The CDC has published a toolkit of interventions to improve ventilation in buildings, many of which are easily achievable at relatively low cost.6 Health Canada recently updated its guidelines for assessing and improving residential indoor air quality using carbon dioxide levels7 — an easily measured proxy for ventilation. Similar guidelines are urgently needed for other indoor spaces, in particular our schools;8 in addition to protecting against airborne transmission, there is a growing body of literature on the general health and cognitive benefits of improved ventilation. Health care staff involved in prolonged, close-proximity patient care have been shown to be at particularly high risk of acquiring SARS-CoV-2 infection, despite contact and droplet PPE.9 Large outbreaks in long-term care homes have been a perfect storm: multiple COMMENTARY HEALTH SERVICES