Résumé
Background Twenty-one months into the pandemic, the extent to which young children get infected and transmit SARS-CoV-2 in school settings remains controversial, in particular with variants of concern. We report a prospective epidemiological, virological and serological investigation of a SARS-CoV-2 outbreak in a primary school in Geneva, Switzerland, in April-May 2021. Methods This outbreak investigation is part of a longitudinal, prospective, primary school-based surveillance study (SEROCoV-Schools). It involved repeated testing of pupils and teachers and household members of participants who tested positive. Rapid antigen tests and/or real-time reverse transcription polymerase chain reaction were performed at Day 0-2 and Day 5-7; serologies on dried capillary blood samples were performed at Day 0-2 and Day 30. Contact tracing interviews and SARS-CoV-2 whole genome sequencing were carried out for positive cases. Results This SARS-CoV-2 outbreak caused by the Alpha variant involved 20 children aged 4 to 6 years from 4 classes, 2 teachers and 3 household members. Infection attack rates were between 11.8 and 62.0% among pupils from the 4 classes, 22.2% among teachers and 0% among non-teaching staff. Secondary attack rate among household members was 10.7%. Symptoms were reported by 63% of infected children, 100% of teachers and 66.7% of household members. All analysed sequences but one showed 100% identity. Serological tests detected 8 seroconversions unidentified by SARS-CoV-2 virological tests. Conclusions This study confirmed child-to-child and child-to-adult transmission of the infection. SARS-CoV-2 can spread rapidly between children and adults in school settings, and is thereby introduced into households. Effective measures to limit transmission in schools have the potential to reduce the overall community circulation.