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.