# Clinical Severity and Hospitalization Burden of Influenza Subtypes: A Cross-Sectional Analysis From a Tertiary Care Center in North India

## TL;DR

This study analyzed influenza cases in North India to compare clinical severity and hospitalization rates among different subtypes, finding that age and breathing difficulties were key factors for hospitalization, not the virus subtype.

## Contribution

The study provides new insights into the clinical burden of influenza subtypes in North India, emphasizing age and symptoms as predictors of hospitalization rather than subtype-specific severity.

## Key findings

- Influenza A(H3N2) was the most common subtype, but hospitalization rates did not differ significantly between subtypes.
- Shortness of breath was the strongest predictor of hospitalization, followed by age under 5 or over 59 years.
- A(H1N1)pdm09 infected more individuals aged 41-59 years compared to A(H3N2).

## Abstract

Introduction

Influenza viruses cause a significant annual burden of respiratory illness. While virological surveillance tracks circulating subtypes, data on the comparative clinical severity and healthcare burden imposed by different influenza subtypes in North India are limited. This study aimed to compare the clinical presentation and hospitalization rates associated with influenza A(H1N1)pdm09, A(H3N2), and B/Victoria lineages and to identify factors associated with hospitalization.

Methodology

A cross-sectional analysis was conducted on 598 laboratory-confirmed influenza cases identified through prospective surveillance of 7,231 patients with influenza-like illness (ILI) from July 2019 to June 2020. Subtyping was performed using real-time reverse transcription-polymerase chain reaction (rRT-PCR). Data on demographics, clinical symptoms, and hospitalization status were collected. Associations between subtypes and clinical outcomes were analyzed using the chi-square test, and factors associated with hospitalization were identified using binary logistic regression.

Results

Influenza A(H3N2) was the predominant subtype, accounting for 379 (63.4%) cases, followed by A(H1N1)pdm09 with 121 (20.2%) and influenza B/Victoria with 98 (16.4%). The overall hospitalization rate was 223 (37.3%). Hospitalization rates did not differ significantly between subtypes. The presence of shortness of breath was the strongest independent predictor of hospitalization (OR: 9.21, 95% CI: 6.23-13.62, p < 0.001). Patients under 5 years (OR: 2.15, 95% CI: 1.21-3.82, p = 0.009) and those over 59 years (OR: 1.82, 95% CI: 1.11-2.98, p = 0.018) also had significantly higher odds of hospitalization. A(H1N1)pdm09 infected a significantly higher proportion of individuals aged 41-59 years (31, 25.6%) compared to A(H3N2) (66, 17.4%; p = 0.037).

Conclusions

The presence of dyspnea and extremes of age were the primary drivers of hospitalization in this influenza cohort, not the infecting viral subtype. These findings underscore the critical importance of closely monitoring patients with respiratory distress and prioritizing vaccination for the very young and elderly. Hospital preparedness for seasonal influenza should focus on managing respiratory complications across all age groups, irrespective of the dominant circulating strain.