Abstract
In fall 2021 in Finland, the recommendation to use face masks in schools for pupils ages 12 years and above was in place nationwide. Some cities recommended face masks for younger pupils as well. Our aim was to compare COVID-19 incidence among 10–12-year-olds between cities with different recommendations on the use of face masks in schools. COVID-19 case numbers were obtained from the National Infectious Disease Registry (NIDR) of the Finnish Institute for Health and Welfare, where clinical microbiology laboratories report all positive SARS-CoV-2 tests with unique identifiers in a timely manner, including information such as date of birth, gender, and place of residence. The NIDR is linked to the population data registry, enabling calculation of incidences. We compared the differences in trends of 14-day incidences between Helsinki and Turku among 10–12-year-olds, and for comparison, also among ages 7–9 and 30–49 by using joinpoint regression. According to our analysis, no additional effect seemed to be gained from this, based on comparisons between the cities and between the age groups of the unvaccinated children (10–12 years versus 7–9 years).

Competing Interest Statement
The authors have declared no competing interest.

Funding Statement
This study did not receive any funding

https://www.medrxiv.org/content/10.1101/2022.04.04.22272833v1