question: BackgroundMask wearing contributes to the reduction of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In Switzerland, compulsory use of masks was introduced in indoor public spaces and later in schools. In the canton of Zurich, masks were introduced for secondary school children (grades 7-9) from November 2020, and for primary school children (grades 4-6) from February 2021- along with other protective measures against SARS-CoV-2. This study explored perceptions towards the usefulness of masks in school and public in a cohort of children and adolescents in the canton of Zurich, Switzerland, in January - May 2021. MethodsSchool children aged 10 to 17 years enrolled in Ciao Corona, a prospective school-based cohort study, responded to nested online surveys between January 12 to March 24 2021 (Q1) and March 10 to May 16 2021 (Q2). Secondary school children were surveyed at Q1 and Q2, and primary school children at Q2 only. Surveys for parents and their children included questions on childrens perception of the usefulness of masks and mask wearing behavior. Associations between perceived usefulness of masks and childs school level, gender, and parents educational attainment were analyzed with Pearsons and McNemars chi-squared tests. Free-text comments provided by children were classified into categories of expressed attitude towards mask wearing. Results595 (54% girls) and 1118 (52% girls) school children responded to online questionnaires at Q1 and Q2, respectively. More than half of school children perceived masks to be useful at school (Q1:60% and Q2:57%) and in public (Q1:69% and Q2:60%). Girls perceived masks as useful more often than boys (at Q2 at school: 61% versus 53%, in public: 64% versus 57%), and children of parents with high educational attainment more often than those of parents with lower educational attainment (at Q2 at school: 61% versus 49%, in public: 63% versus 54%). There were no differences in the perceived usefulness of masks among children in primary versus secondary school. At Q1 and Q2 each, about 20% of children provided individual statements about masks, of which 36% at Q1 and 16% at Q2 reported side-effects and discomfort such as skin irritations, headache or difficulties breathing during physical education. ConclusionApproximately 60% of school children perceived masks at school and in public places as useful. A small but non-negligible proportion of children reported discomfort and side-effects that should be considered to ensure high adherence to mask wearing among school children. Trial registrationClinicalTrials.gov NCT04448717 https://clinicaltrials.gov/ct2/show/NCT04448717 CONTRIBUTION TO THE FIELD STATEMENTWorldwide about 150 countries fully closed their schools at some point during the coronavirus pandemic, while other countries - such as Switzerland - kept schools open almost all the time. However, among other protective measures, children in secondary school (aged approximately 14-16 years) had to wear masks since November 2020, and older children in primary school (aged 11-13 years) - since February 2021. As part of the large study Ciao Corona based in schools in Switzerland, we wanted to learn how children perceive the usefulness of masks in school and public. Children and their parents completed questionnaires in January-March (595 secondary school children) and March-May 2021 (1118 secondary and primary school children). We found that about 60% of children perceived masks to be useful at school and in public. Girls perceived masks as useful more often than boys, and children of parents with university or college education more often than those of parents with lower education. About 7- 9% of children reported side-effects and discomfort such as skin irritations, headache or difficulties breathing during physical education. Although side-effects were not frequently reported, they should be considered to ensure high adherence to mask wearing among school children.