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SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions

By Jennifer Villers, Andre Henriques, Serafina Calarco, Markus Rognlien, Nicolas Mounet, James Devine, Gabriella Azzopardi, Philip Elson, Marco Andreini, Nicola Tarocco, Claudia C Vassella, Olivia Keiser

Posted 20 Aug 2021
medRxiv DOI: 10.1101/2021.08.17.21262169

AO_SCPLOWBSTRACTC_SCPLOWO_ST_ABSBackgroundC_ST_ABSIndoor aerosol transmission of SARS-CoV-2 has been widely recognized, especially in schools where children remain in close proximity 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.

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