Seasonal Trends, Subtype Distribution and Demographic Burden of Influenza Virus Infections: A Three-Year Surveillance Study

Abstract

Background: Influenza remains a significant public health concern, causing seasonal outbreaks with variable intensity and severity. The evolution of circulating subtypes and their differential impacts across age groups and clinical presentations necessitate ongoing surveillance to guide vaccination and treatment strategies.

Objectives: To examine the epidemiological characteristics, subtype distribution, and demographic burden of influenza virus infections over three consecutive influenza seasons (2022–2025), and to compare findings with national and global data.

Methods: A prospective observational study was conducted on 797 patients presenting with Influenza-like Illness (ILI) or Severe Acute Respiratory Infection (SARI). Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to detect and subtype influenza viruses. Demographic, clinical, and virological data were analysed year-wise, by subtype, sex, age group, and clinical severity. Comparative references were drawn from studies by WHO GISRS, ICMR, CDC, and peer-reviewed publications.

Results: Overall Influenza positivity was found to be 27.4%. Yearly Influenza positivity rose from 18.8% (2022–2023) to 36.6% (2023–2024) and it slightly declined to 31.3% (2024–2025). The >1–5 years age group consistently had the highest burden with positivity of 45.3–52.1%. Male patients accounted for 63–71% of total cases. Subtype dominance shifted yearly: Influenza B (Victoria) was predominant in 2022–2023 (76.5%) and 2024–2025 (60.7%), while H1N1pdm09 dominated in 2023–2024 (94.6%). H3N2 resurged in 2024–2025 (38.0%). No Influenza B Yamagata lineage was detected over the past three years. Influenza A subtypes were significantly associated with SARI presentations (H1N1pdm09: 87.7% SARI), while Influenza B also contributed notably to severe illness. Rural areas showed higher positivity (36%) than urban (22.2%).

Conclusion: The study highlights the dynamic epidemiology of influenza with seasonal subtype shifts, age and sex disparities, and a consistent burden among young children. The absence of Influenza B Yamagata over three years suggests its continued global dormancy or extinction. Occurrence of around 30% of Influenza positivity among ARIs cases every year with the most affected age group being children aged 1-5 years and adults >35-60 years, stringent strategies need to be formulated like starting Flu clinics, active and passive surveillance of Influenza cases before and during Influenza seasons, and implementation of Influenza vaccine programs, especially in children.