The recent popularity (1) of the application of James Reason's Swiss cheese model of accident causation to COVID-19 transmission falls short in one important regard—the successive “layers” of defense are too easily perceived as equally effective to control the hazard. A preponderance of evidence now demonstrates that this is not the case (2, 3). Inadequacy in defenses and differential risk (e.g., fabric face coverings as compared to filtering respirators, and aerosol “super-spreading” as compared to droplet or fomite transmission) requires a different approach to conceptualizing COVID-19 risk reduction—particularly as COVID-19 continues to devastate the developing world, new and more transmissible variants emerge, vaccines are not yet equitably available across the globe, and eradication of SARS-CoV-2 is decreasingly likely (4). Additionally, framing defensive layers as equally effective may poses challenges to the adoption of more effective mitigation strategies (e.g., vaccination), when the adoption of seemingly equivalent protections may be preferable (5).

The hierarchy of controls is a framework employed in occupational safety and health to better understand the relative effectiveness of different strategies for risk reduction, and to help determine how to implement feasible and effective solutions (6). The model (Figure 1) is represented as an upside-down pyramid, with five categories represented in descending order of effectiveness: elimination, substitution, engineering controls, administrative controls, and personal protective equipment (PPE). And, while developed to better manage exposures to occupational hazards and protect workers, the model has broad applicability in helping healthcare workers, policy makers, and the public better understand the relative effectiveness of strategies to prevent the transmission of an airborne infectious virus like SARS-CoV-2, and the paradigm is increasingly being adopted to conceptualize COVID-19 risk reduction (7, 8). The fundamental idea behind the hierarchy is that, while different hazard controls are effective at minimizing risk, those at the top of the model are more protective than those at the bottom. As in occupational safety and health, employing the most effective methods first and most frequently can best minimize the risk of COVID-19 transmission.

Recent revisions to Centers for Disease Control and Prevention guidance on transmission notes “infections with respiratory viruses are principally transmitted through three modes: contact, droplet, and airborne.” (9) Contact or “fomite” transmission occurs through touching an infectious person or a surface or item that is contaminated with virus. Droplet transmission occurs through exposure to virus-containing respiratory droplets directly from an infectious person to a susceptible person at close range. Airborne transmission occurs through exposure to smaller virus-containing droplets and particles that float in the air, are highly concentrated close to the person who exhaled them and can remain suspended in air for many seconds to hours and accumulate in the air of poorly ventilated spaces. A key weakness in early COVID-19 response was in treating these hazards as equivalent; recent research has demonstrated comparatively low risk of contact transmission (10), and the World Health Organization has noted that despite evidence of the survival of SARS-CoV-2 on certain surfaces, no reports have directly demonstrated fomite transmission (11). Emerging evidence, on the other hand, suggests significant risk associated with close-contact transmission over short distances and in poorly ventilated spaces, with airborne transmission making the largest contribution (12, 13). Though protecting against each of these hazards is necessary, currently employed controls are not equally effective against each, and higher-order controls should be prioritized.

As it is increasingly likely that COVID-19 will remain endemic, as more transmissible variants emerge, and as at our current pace we may not vaccinate people in low-income countries until the end of 2022 or beyond (18), our continued reliance on the least effective controls and our continued treatment of controls as substitutes instead of complements—such as reliance on face-coverings or social distancing, but not both—has limited our ability to keep SARS-CoV2 in-check. The hierarchy of controls is an effective model for understanding both the relative effectiveness of hazard minimization, and that while most strategies are necessary few are sufficient to slow the spread of COVID-19.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

NS conceptualized the piece and drafted the first version of the manuscript. NS and DM commented on and edited the draft. Both authors reviewed and agreed with the final version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.