# Seasonal Patterns and Subtype Distribution of Influenza Viruses in a Tertiary Care Hospital in North India: A One-Year Observational Study

## TL;DR

This study identifies biannual influenza virus peaks in North India, highlighting the need for region-specific vaccination strategies.

## Contribution

The study reveals a bimodal influenza seasonality pattern in North India and emphasizes young adult transmission dynamics.

## Key findings

- Influenza positivity peaked during late monsoon (August-September) and late winter (March).
- Influenza A dominated (83.61%) over B (16.39%) in the region.
- Young adults aged 20-40 years had the highest infection burden with male predominance.

## Abstract

Introduction: Influenza is a major global cause of morbidity and mortality, with complex seasonality that varies by geography. Data from tropical regions like India are limited but essential for guiding vaccination and preparedness strategies. This study examined seasonal patterns, subtype distribution, and demographic characteristics of influenza virus infections in North India.

Methods: A laboratory-based observational study was conducted at a tertiary hospital in Jaipur from July 2019 to June 2020. Throat swabs from 7,231 patients with influenza-like illness (ILI) were tested for influenza A and B viruses using real-time reverse transcription-polymerase chain reaction (rRT-PCR). Demographic and clinical data were analyzed using descriptive statistics and chi-square tests.

Results: Of the 7,231 samples tested, 598 were positive for influenza (8.26%). Influenza A was predominant, 500 cases (83.61%), compared to influenza B, 98 cases (16.39%). A distinct bimodal seasonality was observed, with peaks during the late monsoon (August-September 2019; positivity rates of 110/364 (30.22%) and 193/838 (23.03%), respectively) and late winter (March 2020; 216/1536 (14.06%)). No activity was detected from April to June 2020. The highest burden was among adults aged 20-40 years (233/598; 38.96%), with a significant male predominance (349/598; 58.4%; p<0.0001). Hospitalization was required for 223 cases (37.3%).

Conclusion: Influenza in North India follows a biannual pattern, with peaks in the monsoon and winter. The high burden in young adults underscores their role in transmission. These findings emphasize the need for region-specific vaccination timing and continuous surveillance to strengthen public health preparedness.