Introduction: COVID-19 has dramatically influenced respiratory viruses? circulation, in particular Respiratory Syncytial Virus (RSV).

Aims and objectives: Evaluation of bronchiolitis epidemiology and clinical features in 2017-2023 epidemic seasons.

Methods: We performed a retrospective observational study, enrolling 620 children aged ? 12 months, admitted for bronchiolitis at the Department of Maternal Infantile and Urological Sciences of the Sapienza University of Rome from September 2017 to March 2023. Anamnestic, clinical and virological data were obtained from medical records.

Results: After the first year of pandemic, RSV became the most frequent detected virus in children hospitalised for bronchiolitis (Figure 1). We demonstrated a slight increase in the median age in 2021-22: 2.7±2.1 in 2017-18, 2.8±1.9 in 2018-19, 2.9±4.4 in 2019-20, 3.2±0.5 in 2021-22 and 2.9±2.2 in 2022-23 (p<0.001). Finally, we found an increase of severe clinical presentations and need of oxygen supplementation in 2022-23: 36.6% in 2017-18, 37.1% in 2018-19, 40.6% in 2019-20, 37.9% in 2021-22 and 60.1% in 2022-23 (p<0.001).

Conclusions: We can speculate that Covid-19 pandemic altered the epidemiology of viral respiratory infections due to viral interference, restrictions and immunological debt. This could have led to an increased median age in 2021-22 and an increased clinical severity in 2022-23.