Thursday, May 25, 2023

PLoS Med.: SARS-CoV-2 Transmission With and Without Mask Wearing or Air Cleaners in Schools in Switzerland


Although the elderly were hit far harder by COVID than the young, thousands of infected children were hospitalized, and nearly 2000 kids (age<18) died in the U.S. alone. Children also served as potential conduits for the virus into the home, where older - often more vulnerable - house members were likely exposed. 

Hopefully the worst of COVID is behind us, but the next pandemic may not go quite as easy on children or adolescents.  

While the use of face masks in public is highly divisive for some people, we continue to see strong evidence that when properly and consistently used, they work to reduce the spread of respiratory diseases (full disclosure: I'm still wearing a mask in crowded indoor venues).  


Today we've a new study out of Switzerland, published in PLoS Medicine, that measured the amount of virus in the air in classrooms of 2 secondary schools over 7 week period during an early Omicron wave, comparing the viral load between no interventions, the wearing of masks, and the use of air cleaners.

Compensating for day-to-day changes, the authors state:

Analyses of environmental changes were adjusted for different ventilation, the number of students in class, school and weekday effects. We modeled disease transmission using a semi-mechanistic Bayesian hierarchical model, adjusting for absent students and community transmission.

What they discovered was that the wearing of masks indoors was associated with a dramatic decrease (69%) in average bioaerosol levels while the use of air cleaners only produced a 39% decrease. 

I've only reproduced the Abstract and author's summary below, so follow the link to read it in its entirety.  I'll have a brief postscript after the break. 

SARS-CoV-2 transmission with and without mask wearing or air cleaners in schools in Switzerland: A modeling study of epidemiological, environmental, and molecular data

Nicolas Banholzer , Kathrin Zürcher , Philipp Jent, Pascal Bittel, Lavinia Furrer, Matthias Egger, Tina Hascher, Lukas Fenner

Published: May 18, 2023



Growing evidence suggests an important contribution of airborne transmission to the overall spread of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), in particular via smaller particles called aerosols. However, the contribution of school children to SARS-CoV-2 transmission remains uncertain. The aim of this study was to assess transmission of airborne respiratory infections and the association with infection control measures in schools using a multiple-measurement approach.

Methods and findings

We collected epidemiological (cases of Coronavirus Disease 2019 (COVID-19)), environmental (CO2, aerosol and particle concentrations), and molecular data (bioaerosol and saliva samples) over 7 weeks from January to March 2022 (Omicron wave) in 2 secondary schools (n = 90, average 18 students/classroom) in Switzerland. We analyzed changes in environmental and molecular characteristics between different study conditions (no intervention, mask wearing, air cleaners). Analyses of environmental changes were adjusted for different ventilation, the number of students in class, school and weekday effects. We modeled disease transmission using a semi-mechanistic Bayesian hierarchical model, adjusting for absent students and community transmission.

Molecular analysis of saliva (21/262 positive) and airborne samples (10/130) detected SARS-CoV-2 throughout the study (weekly average viral concentration 0.6 copies/L) and occasionally other respiratory viruses. Overall daily average CO2 levels were 1,064 ± 232 ppm (± standard deviation). Daily average aerosol number concentrations without interventions were 177 ± 109 1/cm3 and decreased by 69% (95% CrI 42% to 86%) with mask mandates and 39% (95% CrI 4% to 69%) with air cleaners.
Compared to no intervention, the transmission risk was lower with mask mandates (adjusted odds ratio 0.19, 95% CrI 0.09 to 0.38) and comparable with air cleaners (1.00, 95% CrI 0.15 to 6.51).

Study limitations include possible confounding by period as the number of susceptible students declined over time. Furthermore, airborne detection of pathogens document exposure but not necessarily transmission.


Molecular detection of airborne and human SARS-CoV-2 indicated sustained transmission in schools. Mask mandates were associated with greater reductions in aerosol concentrations than air cleaners and with lower transmission. Our multiple-measurement approach could be used to continuously monitor transmission risk of respiratory infections and the effectiveness of infection control measures in schools and other congregate settings.

Author summary

Why was this study done?
  • Public health authorities worldwide closed businesses and schools during the Coronavirus Disease 2019 (COVID-19) pandemic.
  • The closure of schools has been most intensely debated.
  • The contribution of school children to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) transmission and the role of school rooms remain unknown.
What did the researchers do and find?
  • We used molecular, environmental, and epidemiological data to understand the transmission of the virus causing COVID-19 (SARS-CoV-2) in 2 secondary schools (90 students) in Switzerland in the presence and absence of mask wearing and air cleaners.
  • We detected SARS-CoV-2 in aerosols in the air and saliva samples from the students throughout the study.
  • Aerosol and particle concentrations were on average 70% lower with mask mandates and 40% lower with air cleaners.
  • The transmission model estimated that between 2 and 19 infections could be avoided during the study period with mask wearing.
What do these findings mean?
  • Molecular analyses indicated sustained airborne SARS-CoV-2 transmission.
  • Mask wearing may be more effective than air cleaners in reducing aerosol concentrations and transmission of SARS-CoV-2.
  • This approach can be used to assess transmission dynamics and the effectiveness of infection control measures in reducing transmission of respiratory infections during future epidemics.

         (Continue . . . )


Despite millions of COVID deaths, SARS-CoV-2 was not the worst that nature could have thrown at us. Its overall CFR (Case Fatality Rate) was about 1%, and was mostly concentrated among the elderly.

A 1918-like flu pandemic might have double or triple that fatality rate, and skew towards a younger demographic. An H5 or H7 avian flu pandemic, or MERS-CoV, or Virus X could conceivably be even worse. 

Studies like this one continue to show the value of basic NPIs (Non-pharmaceutical Interventions) - like face masks (and to a lesser extent, air filtration) - which can not only reduce infection risks, they can help society remain open and functioning as much as possible during a public health crisis.