Introduction: Influenza B virus (IBV) causes respiratory tract infections with mild, moderate or life-threatening symptoms. This study describes the epidemiology of IBV infection in Rio Grande do Sul (RS), Brazil, along 17 years.

Method: Nasopharyngeal samples were collected from outpatients presenting acute respiratory illness (ARI) between 2003 and 2019, and from inpatients with severe acute respiratory infection (SARI) from 2009 to 2019. IBV was detected by immunofluorescence assay or RT-qPCR; demographic and clinical data were analyzed.

Results: In total, 48,656 cases of respiratory infection were analyzed, of which 20.45% were ARI, and 79.46% were SARI. Respiratory viruses accounted for 22.59% and 37.47% of the cases of ARI and SARI, respectively. Considering respiratory viral infections, 17.10% of ARI and 3.06% of SARI were associated to IBV. IBV circulated year-round in RS, with an increase in autumn and winter, peaking in July (p=0.005). IBV infection showed association with age, and most outpatients positive for IBV were between 10-49 years old, whereas IBV infection in SARI affected mainly individuals ≤1 year or ≥60 years old. No significant association was found between sex and IBV infection. Coryza, sore throat and myalgia were associated with ARI (p<0.001). Moreover, 3.18% of the deaths associated with respiratory virus infection were positive for IBV; notably, cardiopathy (p<0.001), metabolic disease (p<0.001), and smoking (p=0.003) were associated to fatality in IBV infection.

Conclusions: IBV is an important cause of severe respiratory infections, and fatality risk is high in individuals with cardiopathy and metabolic diseases.