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

Influenza, commonly known as the flu, is a highly transmissible viral illness that targets the respiratory passages within the lungs and impacts a large segment of the global population. The clinical manifestations of influenza virus infection vary significantly, ranging from mild or symptomless cases to moderate illness or severe pneumonia. This study aimed to investigate the prevalence, clinical characteristics, and seasonal trends of Influenza B virus (IBV) among individuals presenting with influenza-like illness (ILI) and severe acute respiratory infections (SARI) in Dibrugarh, Assam, Northeast India. Conducted from March 2014 to March 2025 at the ICMR-Regional Medical Research Centre, Dibrugarh, the study analyzed 22,670 nasopharyngeal/throat swab specimens collected from both hospitalized and outpatients individuals spanning various age groups. Samples were processed for ribonucleic acid (RNA) extraction and analyzed for IBV and its subtypes using TaqMan-based real-time RT-PCR. Among the tested specimens, 4.52% (n=1025) were IBV-positive, with all cases identified as the Victoria lineage. Children aged 0–5 years showed the highest prevalence rate of 1.56% (355/22670), followed by 6-17 years with 1.46% (333/22670), while individuals over 50 years showed the lowest detection of 0.10% (24/22670). Males accounted for 2.32% and females 2.19% of total positives. The majority of IBV detections were among ILI cases (6.5%), compared to hospitalized SARI cases (1.35%). Clinical symptoms frequently associated with IBV positive cases included fever (n=773), cough (n=728), and rhinorrhoea (n=548). Seasonal analysis revealed recurrent IBV surges during and after the monsoon months (July to October), with notable peaks in August to September 2016, July to September 2019, and September to October 2021. The overall prevalence shows a irregular pattern of IBV infection, with pronounced surges in 2014, 2016, and 2021, and a moderate rise in 2019. These increases tend to recur every 2–3 years, suggesting a potential periodic or intermittent pattern in IBV activity rather than a stable annual occurrence. Surveillance data between 2022 and 2025 indicated no reported deaths among SARI cases in the study population. The findings underscore a clear age-specific exposure, seasonality, and clinical presentation of IBV in Northeast India, emphasizing the importance of continuous surveillance and timely public health interventions, particularly among young children.