Procedures
Absences from school attendance were notified by the tutor/teacher to the study researchers. We proceed to collect epidemiological and clinical/diagnostic data within the first 48 hours through a case report form deposited on the digital platform REDCap©. All the absences (cases) were followed-up through telephone calls made by health professionals until their return to the school. RTI was registered according to the symptoms described during the telephonic interview and confirmed through the computerised health record programme of the Health Ministry (eCAP).
Sensors to monitor the CO2 concentrations (DIOXCARE DX700 PDF, Smartcare Services, Spain) were installed in classrooms, recording data every 10 minutes. For the analysis we only used the measurements taken during the time in which the students were in the classroom. We obtained a median [IQR] of CO2 levels which was used to conduct our analyses. All the participating schools received an operational protocol to install the sensor correctly and download data weekly.