Seasonal Influenza Vaccination and Disease Severity among Hospitalized Adults in South Kerala

Background: Influenza is an acute viral respiratory illness and a major cause of hospitalization, particularly among elderly individuals and those with chronic comorbidities. Disease severity varies widely, ranging from mild illness to severe respiratory and systemic complications. Seasonal influenza vaccination remains the most effective preventive strategy; however, its uptake and association with disease severity among hospitalized patients in India are not well documented. Objectives: To evaluate the association between seasonal influenza vaccination and disease severity among hospitalized laboratory-confirmed influenza patients. Secondary objectives were to describe the demographic and clinical profile of patients and to estimate influenza vaccination coverage in the study population. Materials and Methods: This hospital-based prospective observational study was conducted among adults with laboratory-confirmed influenza admitted to a tertiary care center in South Kerala between 1 July and 31 December 2025. Data on demographics, comorbidities, clinical features, radiological findings, vaccination status, management, complications, and outcomes were collected prospectively. Disease severity was assessed based on the occurrence of complications, requirement for oxygen therapy, intensive care unit (ICU) admission, and in-hospital mortality. Associations were analyzed using Chi-square or Fisher’s exact test, with P < 0.05 considered statistically significant. Results: Fifty-two patients were included (median age: 69 years); 42.3% were aged above 70 years, and 53.8% were female. Only 38.5% had received seasonal influenza vaccination. Complications occurred in 53.8% of patients, predominantly respiratory, followed by cardiac complications. Vaccinated patients had a significantly lower incidence of complications compared with unvaccinated patients (35% vs. 65.6%; P = 0.031). ICU admission was required in 13.5% of cases. Overall outcomes were favorable, with 98.1% discharged and a mortality rate of 1.9%. Conclusion: Seasonal influenza vaccination was associated with reduced disease severity and fewer complications among hospitalized influenza patients. Strengthening vaccination coverage among elderly individuals and those with chronic comorbidities is essential to reduce influenza-related morbidity.