Laboratory diagnosis and circulation of respiratory syncytial virus (A and B subgroups) and influenza virus A (H1 and H3 subtypes) and B in a three-winter season (2016-17 to 2018-19) hospital-based survey in northern Italy

Background: Respiratory syncytial virus (RSV), Influenza A (IAV) and B (IBV) virus are among the leading causes of viral upper and lower respiratory infections and a significant cause of hospitalization and even death in ‘at-risk’ individuals, such as children and older adults. We analysed a three-winter seasonal circulation of RSV (A and B subgroups), IAV (H1 and H3 subtypes) and IBV in the population with influenza-like illness (ILI) attending to the University Hospital of Parma, Italy. Materials/methods: Respiratory samples (n=2066), collected in three winter seasons (December 2016-March 2019) from 1875 patients with ILI, were analyzed by conventional and molecular methods for viral detection and subtyping at the Virology Unit of the University Hospital of Parma, Italy. Results: Among the 810 RSV and/or IV positive samples, 22.8% were RSV A, 34.9% RSV B, 34.8% IAV and 7.4% IBV. As to RSV subgroups, RSV A prevailed in 2016/2017, and was reversed by RSV B in the last two seasons. RSV was identified in children ranged from 1 month to 1 year and in elderly (>50 years). Concerning IV, all the epidemic seasons were characterized by the prevalence of IAV; IAV-H3 dominated in 2016/17, while all samples resulted positive for IAV-H1 in 2017/2018; finally, both subtypes co-circulated in the 2018/19. As regards IBV circulation, a significant increase of IBV was detected only in the 2017/2018, vs a negligible number of cases in the 2016/2017 and no cases in the 2018/2019 winter seasons. Both IAV and IBV infections were most prevalent among children ranged from 1 to 6 years and ≥ 50 year-old adults. Conclusions:This study represents a useful tool for the surveillance of viral infectious agent circulation and variability, as the considered population is mostly represented by hospitalized pediatric and elderly patients who more often can develop adverse events upon RSV or IV infection. Remarkably, RSV A responsible for the most severe clinical pictures in infants was reversed by RSV B in the two last winter seasons, possibly related to new genetic variants. Also of interest, IBV peaked only in 2017/18, according to the European Centre for Disease Prevention and Control data.