Unravelling the Role of the Mandatory Use of Face Covering Masks for the Control of SARS-CoV-2 in Schools: A Quasi-Experimental Study Nested in a Population-Based Cohort in Catalonia (Spain)

Background: Mandatory use of face covering masks (FCM) had been established for children aged six and above in Catalonia (Spain), as one of the non-pharmaceutical interventions aimed at mitigating SARS-CoV-2 transmission within schools. To date, the effectiveness of this mandate has not been well established. The quasi-experimental comparison between 5 year-old children, as a control group, and 6 year-old children, as an interventional group, provides us with the appropriate research conditions for addressing this issue.Methods: We performed a retrospective population-based study among 599,314 children aged 3 to 11 years attending preschool (3-5 years, without FCM mandate) and primary education (6-11 years, with FCM mandate) with the aim of calculating the incidence of SARS-CoV-2, secondary attack rates (SAR) and the effective reproductive number (R*) for each grade during the first trimester of the 2021-2022 academic year, and analysing the differences between 5-year-old, without FCM, and 6 year-old children, with FCM.Findings: SARS-CoV-2 incidence was significantly lower in preschool than in primary education, and an age-dependent trend was observed. Children aged 3 and 4 showed lower outcomes for all the analysed epidemiological variables, while children aged 11 had the higher values. Six-year-old children showed higher incidence than 5 year-olds (3·54% vs 3·1%; OR: 1·15 [95%CI: 1·08-1·22]) and slightly lower but not statistically significant SAR and R*: SAR were 4·36% in 6 year-old children, and 4·59% in 5 year-old (IRR: 0·96 [95%CI: 0·82-1·11]); and R* was 0·9 and 0·93 (OR: 0·96 [95%CI: 0·87-1·09]), respectively.Interpretation: FCM mandates in schools were not associated with lower SARS-CoV-2 incidence or transmission, suggesting that this intervention was not effective. Instead, age-dependency was the most important factor in explaining the transmission risk for children attending school.Funding Information: CP and SA received funding from Ministerio de Ciencia, Innovación y Universidades and FEDER, with the project PGC2018-095456-B-I00.Declaration of Interests: The authors declare that they have no conflict of interests.Ethics Approval Statement: The study was evaluated and approved by the Clinical Research Ethics Committee of the IDIAP Jordi Gol, Reference 21/018-PCV. This research was based on the agreement established in Regulation 2016/679 of the European Parliament and the Council of Europe of 27 April 2016 on Data Protection, and Organic Law 3/2018 of December 5 on the protection of personal data and the guarantee of digital rights.