Introduction
Different public health measures were implemented during the COVID-19 pandemic to maintain the schools open1,2. Regarding COVID-19 vaccination, at the beginning of this study, more than 92% and 40% of population older than 12 years and children aged 5-11 years in Spain was fully vaccinated against COVID-19, respectively3.
Ventilation was proposed as one of the main strategies to reduce aerosol transmission for SARS-CoV-2 during the pandemic. A study from Germany assessed the efficiency of air purifiers in reducing aerosols in high-school classrooms4. However, no clinical endpoints were investigated and, therefore, a very low certainty of evidence was graded by a Cochrane review5. In an Italian study, the authors concluded that ventilation reduced the likelihood of SARS-CoV-2 infection in classrooms by 80%6. Moreover, a survey study from the US reported that elementary schools with face masks and ventilation strategies in place had lower SARS-CoV-2 incidence rates7. In a Lancet Task Force Commission review8, the authors concluded that improving building ventilation systems may carry benefits beyond protection from COVID-19. However, there is scarce evidence that demonstrate the association of epidemiological and environmental risk factors, including CO2 concentrations, with the incidence of respiratory tract infections (RTI) in children who are attending schools.
The present study is part of COVID-19 Sentinel Schools Network of Catalonia (CSSNC), a project including 23 schools and 5687 students, teachers and other school staff with the aim to monitor SARS-CoV-2 and other respiratory viruses, their determinants and preventive measures in Catalonia9. The objectives of this study are to assess the potential association of demographic and epidemiological factors, including the indoor environmental conditions (CO2levels) in the classrooms, with students’ school absence due to RTI.