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Review 3: "SARS-CoV-2 Aerosol Transmission in Schools: The Effectiveness of Different Interventions"

Reviewers find this a straightforward modelling study, though ask for clarification on some assumptions and emphasize that the results may apply only to naturally-ventilated classrooms.

Published onSep 19, 2021
Review 3: "SARS-CoV-2 Aerosol Transmission in Schools: The Effectiveness of Different Interventions"
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key-enterThis Pub is a Review of
SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions

AbstractBackgroundIndoor aerosol transmission of SARS-CoV-2 has been widely recognized, especially in schools where children remain in closed indoor spaces and largely unvaccinated. Measures such as strategic natural ventilation and high efficiency particulate air (HEPA) filtration remain poorly implemented and mask mandates are often progressively lifted as vaccination rollout is enhanced.MethodsWe adapted a previously developed aerosol transmission model to study the effect of interventions (natural ventilation, face masks, HEPA filtration, and their combinations) on the concentration of virus particles in a classroom of 160 m3 containing one infectious individual. The cumulative dose of viruses absorbed by exposed occupants was calculated.ResultsThe most effective single intervention was natural ventilation through the full opening of six windows all day during the winter (14-fold decrease in cumulative dose), followed by the universal use of surgical face masks (8-fold decrease). In the spring/summer, natural ventilation was only effective (≥ 2-fold decrease) when windows were fully open all day. In the winter, partly opening two windows all day or fully opening six windows at the end of each class was effective as well (≥ 2-fold decrease). Opening windows during yard and lunch breaks only had minimal effect (≤ 1.2-fold decrease). One HEPA filter was as effective as two windows partly open all day during the winter (2.5-fold decrease) while two filters were more effective (4-fold decrease). Combined interventions (i.e., natural ventilation, masks, and HEPA filtration) were the most effective (≥ 30-fold decrease). Combined interventions remained highly effective in the presence of a super-spreader.ConclusionsNatural ventilation, face masks, and HEPA filtration are effective interventions to reduce SARS-CoV-2 aerosol transmission. These measures should be combined and complemented by additional interventions (e.g., physical distancing, hygiene, testing, contact tracing, and vaccination) to maximize benefit.

RR:C19 Evidence Scale rating by reviewer:

  • Potentially informative. The main claims made are not strongly justified by the methods and data, but may yield some insight. The results and conclusions of the study may resemble those from the hypothetical ideal study, but there is substantial room for doubt. Decision-makers should consider this evidence only with a thorough understanding of its weaknesses, alongside other evidence and theory. Decision-makers should not consider this actionable, unless the weaknesses are clearly understood and there is other theory and evidence to further support it.



Overall, I would rate the paper as potentially informative. The results are obvious and confirm previous work, more air exchanges in classrooms yield lower estimates SARCoV-2 airborne particles. However, the article can help public health professionals with risk assessment and implementation of controls to reduce exposure. The heat maps were particularly useful and showed that portable HEPA filtration can provide a comparable reduction in viral concentrations as natural ventilation and can be used in situations where classrooms have window issues and/or the temperature gradients between indoors and outdoors is small. The main limitation of the paper is that the particle estimates have not been validated with any real-world data and rely on simplistic models. Although this is a weakness, this method of infectious disease risk assessment has been utilized for a number of years and helps in the risk prioritization process. Other limitations that should be addressed:

1) the estimates of 35%-80% reduction in particle transmission from mask wearing is high and not based upon data for children. The authors should consider reducing this for more realistic estimates.

2) The study also does not consider removal of particles via a HVAC system.

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