Title: Determinants of school absences due to
respiratory tract infections among children during COVID-19 pandemic: a
cross-sectional study of the Sentinel Schools Network.
Antoni Soriano-Arandes1,●,○, Andreu
Colom-Cadena2,3,●, Anna Bordas2,3,
Fabiana Ganem2,3,4, Lucia Alonso3,5,
Marcos Montoro2,3, Mireia
Gascon6,7,8, Maria Subirana6,7,8,
Ariadna Mas9, Jordi Sunyer6,7,8,
Pere Soler-Palacin1, Jordi
Casabona2,3,4,8 ; on behalf of Sentinel School Network
of Catalonia*
- Paediatric Infectious Diseases and Immunodeficiencies Unit. Children’s
Hospital. Vall d’Hebron Barcelona Hospital Campus, Barcelona,
Catalonia, Spain.
- Centre d’Estudis Epidemiològics sobre les ITS i Sida de Catalunya
(CEEISCAT). Badalona, Spain.
- Institut d’Investigació Germans Trias i Pujol (IGTP), Badalona, Spain.
- Departament de Pediatria, d’Obstetrícia i Ginecologia i de Medicina
Preventiva i de Salut Publica, Universitat Autònoma de Barcelona,
Bellaterra, Spain.
- Fundació Lluita contra les infeccions, Badalona, Spain.
- ISGlobal, Barcelona. Spain.
- Universitat Pompeu Fabra (UPF), Barcelona, Spain
- Centro de Investigación Biomédica en Red de Epidemiología y Salud
Pública (CIBERESP) Instituto de Salud Carlos III, Madrid, Spain.
- Direcció Assistencial d’Atenció Primària i Comunitària, Institut
Català de la Salut, Barcelona, Catalonia, Spain
● Contributed equally to this work with: Antoni
Soriano-Arandes, Andreu Colom-Cadena
○ Corresponding author: Antoni
Soriano-Arandes,toni.soriano@vallhebron.cat.
ORCID: 0000-0001-9613-7228
Keywords: COVID-19, respiratory infections, ventilation,
school, CO2, Indoor air quality
To the editor:
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.
We conducted a cross-sectional study in two different periods, the first
from April 19 to June 21, 2022, and the second from November 8 to
December 21, 2022. The main outcome of the study was the absence of the
child from attending the school in person due to RTI or other
non-respiratory medical causes. The study population was composed of 253
students (4-11 years) belonging to 20 classrooms, 11 in spring 2022 and
9 in autumn 2022, attending 4 and 16 classrooms of preschool (4 to 5
years-old) and primary school (6 to 11 years-old) stages, respectively.
A written informed consent was previously obtained from parents or
guardians of children.
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.
We calculated the RTI incidence per week and for the total study period.
We performed a descriptive analysis of all survey variables, stratifying
them by respiratory-related absence or absence due to other causes.
Additionally, we ran a univariate logistic model to study the
association between the type of absence and each described variable to
obtain the corresponding odds ratios, 95% confidence intervals and
p-values.
Furthermore, we conducted a Latent Class Analysis (LCA) to explore
potential groups of students with similar symptomatology. We started
exploring the optimal number of latent classes, trying two to five
classes, and we selected the best model using the entropy criterion,
which indicates the accuracy of the latent classes, combined with other
goodness of fit criteria such as BIC, cAIC and likelihood ratio. All
these measures suggested that the optimal number of classes was two.
However, we also examined the rest of number of classes to see if their
classification had more clinical significance. All statistical analyses
were performed in R (version 4.2.2).
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) (Figure 1S ). 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.
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.