The added value of diagnostics to characterize age-specific patterns of respiratory viral infections and coinfections and to detect emerging threats.

Background: Pandemic restrictions caused variation in respiratory virus circulation until the winter of 2022/23. The aim of this study was to monitor respiratory virus cases in the 2023/24 epidemic season.

Methods: Children and adults attending Sapienza University Hospital for acute respiratory infections (October 2023-June 2024) were tested for respiratory viruses via molecular methods.

Results: Of the 1121 patients included, 880 (78%) were positive for rhinovirus (HRV, 32%), Influenza A (IAV, 29%), and respiratory syncytial virus (RSV, 28%). RSV is more common in infants, and IAV is more common in adults, whereas HRV is more common in children aged 1-5 years. IAV, RSV and HRV cocirculate in winter; HRV cases also occur in spring, along with Influenza B (IBV) and other viruses. Despite circulating in the same weeks, the number of observed coinfections was much lower than that predicted for IAV and RSV (p <.0001) and lower also for the IAV/IBV, IBV/RSV and RSV/HRV pairs (p <.0001, p =.0059, p =.015, respectively). IAV and RSV cocirculated with different patterns in different age groups. In fact, in children aged 1-5 years, the RSV peak preceded that of IAV, whereas in older age groups, the RSV peak occurred toward the end of IAV circulation. Sequencing of HRV/EV cases in spring revealed 25 HRV genotypes and two EV-C105 cases.

Conclusions: Respiratory viruses can cause age-specific seasonal peaks that are modulated by viral interference phenomena. Molecular diagnostic data should be integrated with surveillance programs to characterize seasonal circulation patterns of common respiratory viruses and to rapidly detect the next pandemic threat.