Discussion
Our findings confirm that the RTI incidence during the study period was very high in children attending in the CNSSC schools, and the main medical cause of school absence. Although differences on school absence due to RTI were observed between spring and autumn, they were not statistically significant and in any case they may be due to the respiratory viruses’ seasonal pattern. In fact, the most important and significantly associated risk factor for RTI was the presence of someone else at home with respiratory symptoms, suggesting that households could be the main setting for initiating of the transmission of RTI.
We observed a slightly association between RTI and median of CO2 levels in classrooms (p=0.07), which is an indicator of the degree of ventilation. However, we cannot exclude other potential factors such as rainfall, ambient temperature, or air pollutants (e.g. PM2.5, NO2, etc.) influencing on this outcome, as suggested by other authors10. To our knowledge, previous studies assessed the CO2concentration as a proxy of ventilation to evaluate the risk transmission of SARS-CoV-2 in schools11,12, but they did not analyse the association between CO2 median values and RTI incidence.
Finally, we studied the symptomatology associated with the school absences through a LCA. The best approach to differentiate RTI from other causes was using two latent classes, and the most frequent symptoms were cough, nasal congestion and fever.
The major strength of this study is our extensive data collection on clinical, epidemiological and environmental factors related to the school and also to the households of the participants. However, there were limitations such as possible incomplete reporting of RTI or insufficient sample size to determine small effect sizes. CO2 concentration was only measured in a selection of classrooms per school, so it may not be representative for the entire study period and school.
In conclusion, RTI incidence was very high during the study period being the most important and significantly associated factor with RTI to have anyone else at home with respiratory symptoms. This suggests that households and not schools could be the key epidemiological factor for initiating the transmission of RTI to the children. Improving household preventive measures could reduce childhood RTI. In the LCA, the most frequent symptoms associated with RTI were cough, nasal congestion and fever. Although we found a slightly association between RTI and reduced ventilation we cannot exclude other potential factors influencing on this outcome. The study has been crucial to assess the feasibility and potential utility of collecting both school absence and morbidity data for further developing a systematic monitoring system.