3.1 Demographic, epidemiological, and microbiological data
Overall, we included overall 982 patients (57.5% males, 41.5% females) hospitalized for bronchiolitis in the study period. During the 2022-2023 season, 333 children (57.1% males, 42.9% females) were admitted.
As shown in Figure 1 , after the anticipated peak of the 2021-2022 season, in 2022-2023 we observed a further increased, but slightly delayed peak with the highest incidence of admissions in December.
We observed a statistically significant increase in the 2022-2023 season in the absolute
number of bronchiolitis hospitalizations reaching a rate of 17*1000 hospitalizations (Figure 2 ). Table 1 shows the main demographic, microbiological, clinical, and outcome details. Similarly, to the previous five years, in the 2022-2023 season, the median age at admission was 78 days. Comorbidities were reported in 37 (12.5%) and among them, 21 (7%) had prematurity (range 26-36 weeks of gestational age, GA), with no significant differences compared to the previous five seasons.
In the last season, RSV was increasingly identified as the causative agent in 75.3% of children (p<0.001) , while SARS-CoV-2 decreased from 4.2% to 1.2%.
We observed an apparent increasing trend of out-seasonal bronchiolitis over the years with a nonnegligible number of admissions in the period May-August 2023 (Figure 3 ). In none of these cases, RSV was found as the causative agent and no patients required admission to the critical care area (ICU or IMCU).