Results
We registered one-hundred and five school absences during the study, 43 in the first study period (spring 2022), and 62 in the last one (autumn 2022) (Supplementary Figure 1 ). Among these, we could obtain data related to the diagnosis in 98 (93%) cases, the rest of them were lost during the follow-up of the absence. Seventy-one absences were respiratory-related and 27 were due to other causes, mainly with gastrointestinal symptoms (66.7%). Among the absences due to RTI, the most were upper RTI (56/71, 78.9%), and only six (8.5%) were confirmed infections, 2 caused by influenza virus and 4 due to SARS-CoV-2 infection. These results represent a RTI incidence of 15.9 and 33.1 cases per 100 population in the first and second study period, respectively. The maximum weekly RTI incidence was of 10.2 cases per 100 population in December 12-18, 2022.
The clinical, epidemiological and environmental characteristics of study sample are summarized in table 1 , categorized by type of absence (respiratory versus non-respiratory). We found a statistically significant association with absences due to RTI when someone else at home had respiratory symptoms (OR=9.12, CI 95%=2.54-33.39). We found a positive association between higher median levels of CO2at class and respiratory-related absences (OR=1.2, CI 95%=0.98-1.46). Moreover, there were more respiratory absences in autumn (OR=2.4, CI 95%=0.97-5.94). However, these last two associations did not reach statistical significance at a level of 0.05; although their p-values were lower than 0.1.
No other epidemiological risk factors were associated with RTI incidence, such as household floor level, number of people living at home, living with smokers, having any comorbidity or being vaccinated against COVID-19.
Finally, in Figure 1 , we present the symptomatology of absences belonging to each of the two latent classes. We can see that in the first cluster, the most frequent symptoms were cough, nasal congestion and fever, whereas in the second one, fever, gastrointestinal symptoms, and fatigue predominated.