Which interventions can help prevent SARS-CoV-2's spread in schools?

With schools and other educational institutions having been closed for much of the school year globally, the need to recommend and evaluate methods to minimize the spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the ongoing coronavirus disease 2019 (COVID-19) pandemic is essential.

Study: SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions Image Credit: Halfpoint/ Shutterstock

Study: SARS-CoV-2 aerosol transmission in schools: the effectiveness of different interventions. Image Credit: Halfpoint/ Shutterstock

A new study shows that leaving the windows open all day is the most effective intervention, with higher efficacy in winter relative to summer. Still, the use of surgical face masks was also very effective. However, combined modality use carried the greatest effectiveness against viral transmission, even when a super-spreader was present.

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.


Children were thought to be relatively less involved in viral transmission because they had less severe infections, with a larger proportion of asymptomatic infections. The subsequent school outbreaks showed that children did drive local spread, as supported by data from England, where the delta variant of SARS-CoV-2 was found to spread the fastest among people up to 24 years of age.

The reasons for this phenomenon include the significantly lower rates of vaccination in this group and the lack of mitigation interventions in schools despite the knowledge that the virus is spread primarily through airborne particles. Such droplets persist in the air near an infected person and accumulate in the absence of ventilation.

Indoor spaces are the places of greatest risk of the spread of infection. Therefore, the virus can spread via indirect and long-range contact. Schools are crowded and badly ventilated, leading to recommendations to follow physical distancing and hand hygiene in schools. These are not effective enough, however, when the long-range viral spread is considered.

Natural ventilation is used where available, but if not, the US Centers for Disease Control and Prevention (CDC) recommend portable filtration devices be used. High-efficiency particulate air (HEPA) filtration systems are recommended since they can remove at least 99.97% of airborne particles with a size of 0.3 microns or more, including bacteria and viruses.

Masks are recommended by the CDC for all children and school staff, but only for the older individuals in the European Union.

What did the study show?

The paper aimed to compare the effectiveness of the previously mentioned interventions to reduce SARS-CoV-2 spread in a standard classroom. The researchers chose to use the COVID Airborne Risk Assessment (CARA) tool.

The factors examined included the viral emission rate from the nose or mouth, the viral concentration in the air during exposure, the absorbed dose of viral particles following inhalation by an exposed host, and the chances of transmission on-site.

The viral concentration depends on the room volume, the occupancy at any given time, the rate of removal of viral particles as influenced by:

  • natural ventilation (determined by the area of openable windows and the indoors-outdoors temperature gradient)
  • the level of air filtration (number and effectiveness of filters)
  • gravity-dependent particle settling
  • the rate of decay of infectious viruses in aerosols over time.

Open windows

The researchers compared the opening of one side of one, two, or six windows. Two different seasons were simulated, spring/summer and winter, with outdoor temperatures of 18°C and 5°C, respectively, with an indoor temperature of 22°C.

When windows were kept open throughout the whole day in spring/summer, the cumulative dose was reduced by two-, three- and seven-fold, with one, two, or all six windows, respectively. If kept open only during breaks, the dose reduction was between 1.1-1.2-fold.

Whole-day opening during winter reduced the cumulative dose by three-, five- and 14-fold with one, two, and six windows, respectively. Practically, this is not possible as the room cannot be heated without unacceptable waste of energy. Thus, the scientists looked at keeping the windows 20 cm open all day instead during winter.

This decreased the cumulative dose by 1.7-, 2.4- and five-fold, with one, two, and six windows cracked open. If windows were opened after each class for ten minutes, the cumulative dose was reduced by about two-fold in summer and winter.

HEPA filtration

With a rate of 5 ACH and two filters per classroom, vs. 2.5 ACH (corresponding to one filter), the results show that the latter has an effect equivalent to having two windows partly open the whole day long during winter. The effect is doubled by using 5 ACH, and further doubled when face masks are used by staff and students.

Combined interventions

When natural ventilation and masking are combined, the cumulative dose is reduced eight-fold than with ventilation alone, for a total reduction of 19-fold below the baseline. When face masks were used with HEPA filtration, a similar reduction was achieved with 2.5 ACH, but an astonishing reduction by ~30-fold at 5 ACH.

A combination of all three brought the reduction to 30-fold and 40-fold for 2.5 and 5 ACH, respectively.

What of a super-spreader?

Research indicates that 80% of secondary spread is due to15% of individuals. With a super-spreader present, the cumulative dose increased five-fold and 16-fold. However, the cumulative dose dropped to below baseline conditions when ventilation with face mask use was combined.

What are the implications?

The study shows that a combined approach can reduce the cumulative dose of viral particles absorbed by an individual below the baseline. Opening the windows only during breaks is ineffective and should not be advised.

The researchers highlight that school-age children and young adults have the highest infection rates with the spread of the delta variant to the UK. Thus, such strategies have proven valuable among unvaccinated and crowded groups. To maximize effects, the group recommends using the aforementioned measures in unison with additional interventions, such as social distancing.

*Important notice


medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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