Fig. 1

Screening algorithm during a possible student residence outbreak. Abbreviations: D1 = day one, as soon as possible after the diagnosis of the index case. D7 = Day seven, seventh day after the day of diagnosis of the index case, Q = Quarantine, T = testing, HRC = high-risk contact. *Residence unit: students sharing a kitchen or sanitary facilities. Screening was as follows: if an index case recently resided in a student residence, all students who structurally/contractually shared either the same kitchen or sanitary facilities with them and who had also resided in the residence in the week leading up to the onset of symptoms or diagnosis of the index, were invited for testing as soon as possible. They were part of the same residence unit. Contacts who were already diagnosed with COVID-19 between 14 and 60 days prior to the index were not eligible for screening. A subset of students in this residence unit was additionally asked to quarantine and undergo a second test on day 7. This subset depended on whether hygiene measures were strictly complied with and whether high-risk contacts (contact for > 15′ at < 1,5 m without face masks, or direct physical contact) could be readily identified through contact tracing. If other living units interacted regularly with the one harboring the initial index, those other units were also invited for a first test. In case additional cases were diagnosed in a particular unit, students belonging to this unit were asked to quarantine and undergo a second test on day seven. The detection of new cases in a unit could lead to additional screening rounds following the same protocol