Temporal trends and burden of influenza B virus lineages in Dibrugarh, Assam (2014-2025): predominance of Victoria with early circulation of Yamagata.

Influenza is a contagious viral illness that primarily affects the respiratory tract and poses a substantial public health burden worldwide. Clinical presentations vary from mild or asymptomatic infection to severe pneumonia. This study investigated the prevalence, clinical characteristics, and seasonal trends of Influenza B virus (IBV) among patients with influenza-like illness (ILI) and severe acute respiratory infection (SARI) in Dibrugarh, Assam. From March 2014 to March 2025, a total of 22,670 nasopharyngeal/throat swab specimens were collected at the ICMR-Regional Medical Research Centre, Dibrugarh. RNA was extracted and screened for IBV using TaqMan-based real-time RT-PCR. Overall, 4.52% (n = 1025) were IBV-positive, with 988 cases of Victoria lineage and 37 cases of Yamagata lineage. Children aged 0–5 years showed the highest prevalence (1.56%), followed by 6–17 years (1.46%), while detection was lowest among individuals > 50 years (0.10%). IBV positivity was slightly higher in males (2.32%) than females (2.19%). Most detection occurred in ILI cases (6.5%) compared with hospitalized SARI cases (1.35%). Common symptoms include fever (n = 773), cough (n = 728) and rhinorrhoea (n = 548). Seasonal peaks were noted during and after the monsoon months (July–October), with surges in 2016, 2019 and 2021, indicating a periodic pattern rather than consistent annual circulation. Importantly, no IBV-related deaths were recorded among SARI cases between 2022 and 2025. These findings highlight age-specific susceptibility, distinct seasonality and characteristic clinical features of IBV in Northeast India, reinforcing the need for continuous surveillance and timely public health measures, especially for young children.