ABSTRACT
Introduction: In Italy, the InfluNet integrated influenza surveillance system, during the 2021-2022 season, in the middle of the SARS-CoV-2 pandemic, was adequately modified and strengthened to support the monitoring of the spread of the SARS-CoV-2 virus. The present work aims to describe the impact of the 2021-2022 flu season on the Italian population, also in the light of the changes introduced in the InfluNet surveillance system. For a complete assessment of the impact of influenza viruses, the influenza vaccination coverage achieved during the season was also evaluated. Materials and methods: InfluNet is based on a network of sentinel physicians made up of general practitioners and pediatricians, recruited by the Italian Regions, who report cases of Influenza Like Illness (ILI) observed among their patients. Sentinel physicians (sentinel flow) and other doctors working in hospitals (hospital flow) also collaborate in the collection of biological samples for the identification of circulating viruses. To evaluate the impact of ILI on the Italian population, overall incidence (per 1,000 assisted) was calculated, as well as stratified by age group. Furthermore, vaccination coverage (%) in the total Italian population and by age group were also calculated. Results: The 2021-2022 season was characterized by limited circulation of flu viruses. At the end of December 2021, the incidence of ILI reached an initial peak of 5.16 cases per 1,000 assisted. A second peak was identified, also very contained, at the end of March 2022, equal to 5.25 cases per 1,000 assisted. Virological surveillance made it possible to characterize the two waves: the first was mainly supported by the circulation of respiratory syncytial virus, especially in children under five years of age, while the second wave was characterized by the circulation of influenza viruses. 14.4% of clinical samples analyzed by InfluNet network laboratories were found to be positive for influenza viruses. Among the positive samples, 82.9% belonged to the A(H3N2) virus and 29.3% tested positive for SARS-CoV-2. Vaccination coverage in the 2021-2022 season, which decreased in all age groups, was 20.5% in the general population and 58.1% in the elderly population. Discussion and conclusions: In Italy, the 2021-2022 season marked the return of the circulation of influenza viruses after the start of the COVID-19 pandemic, though in a moderate intensity. The InfluNet surveillance system is turning into a respiratory virus monitoring system and has made it possible, this season, to characterize the trend of ILI and to attribute the increase in cases to the various respiratory viruses. Vaccination coverage decreased compared to the previous season. The adoption of preventive measures in recent seasons has profoundly changed the epidemiology of ILI and respiratory viruses in terms of intensity and seasonality. The accumulation of susceptible population, especially in the pediatric age groups, has partly allowed a greater circulation of respiratory viruses in the 2021-2022 season and will also presumably allow it in future seasons. It is therefore increasingly important to vaccinate against flu to reduce serious complications and deaths, especially in the fragile population.