Influenza viruses circulation in a tertiary care children hospital in Rome: a comparison between 2022 and the previous 5 years

Stefania Ranno, Luana Coltella, Giulia Linardos, Velia Chiara Di Maio, Luna Colagrossi, Leonarda Gentile, Eugenia Galeno, Marta Luisa Ciofi degli Atti, Sebastian Cristaldi, Alberto Villani, Massimiliano Raponi, Carlo Federico Perno, Cristina Russo

Influenza surveillance aims to determine onset, duration and intensity of the seasonal Influence-like Illness (ILI); data collection begins in the week 42 of a year and ends in the week 17 of the following year. In this observational study, we report the experience of a tertiary care children hospital in Rome about Influenza viruses circulation during the calendar year 2022 (January-December) in comparison with the previous five years (2017-2021), with a special focus on the weeks 18-41, usually not under surveillance. This retrospective study involved 36782 respiratory samples referred to 21354 patients (pts), median age 2.63 years, admitted with respiratory symptoms at Bambino Gesù Children's Hospital in the years 2017-2022. Respiratory viruses were detected by molecular Allplex™ Respiratory Panel Assays (Seegene, Korea). Regarding the pre pandemic years, 2017-2019, distribution of Flu positive patients focused in the first weeks of the year (weeks 1-17). During the pandemic period, Flu was not detected. In 2022, 239 Flu viruses were identified: 37 FluA (weeks 1-17), 29 FluA (weeks 18-41) and 168 FluA and 5 FluB (weeks 42-52). For the year 2022, during the non-epidemic period, the number of Flu viruses detected corresponded to 12.1% of total Flu detected, respect to 0-1.7% for the previous five years (p < 0.001). When compared with pre SARS-CoV-2 pandemic years, our data show a significant increase in Influenza cases during weeks 18-41/2022 and reveal an unexpected summer circulation of these viruses: just weeks 26-30 showed to be influenza virus free. A national year-round Flu surveillance could be useful to understand if changing in influenza epidemiology is transitional or likely to persist in the following years.