Prevalence, Seasonal Distribution, and Subtype Distribution of Influenza A and B among Pediatric Acute Respiratory Illness Cases in a Tropical Monsoon Region: A 22-Month Retrospective Study

Abstract

Influenza viruses are major causes of acute respiratory illness (ARI) in children and a leading reason for hospitalization in low- and middle-income countries (LMICs). Tropical monsoon climates show variable influenza activity compared with temperate regions. We evaluated the prevalence, seasonal trends, and subtype distribution of Influenza A and B among children ≤12 years presenting with ARI over 22 months in Kerala, India. Retrospective cross-sectional study at Government Medical College Kozhikode from 1 January 2024 to 31 October 2025. Medical records of pediatric patients (≤12 years) meeting the World Health Organization (WHO) ARI case definition were reviewed. Respiratory specimens underwent real-time reverse transcription polymerase chain reaction (RT-PCR) for Influenza A and B. Demographic and temporal data were extracted. Descriptive statistics summarized prevalence. Seasonal trends were evaluated month-wise and by predefined seasons (south-west monsoon: Jun–Aug; north-east monsoon/winter: Nov–Jan; other: Feb–May, Sep–Oct). Chi-square test compared seasonal distributions of Influenza A and B. Of 1,584 pediatric ARI cases, 210 (13.3%) were influenza-positive: Influenza A 152 (72.4%) and Influenza B 58 (27.6%). Sub-typing of 33 Influenza A samples showed 42% H1N1 and 58% H3N2. All 14 Influenza B samples sub-typed, belonged to the Victoria lineage. Influenza A peaked during June–August (south-west monsoon) in both 2024 and 2025 (119 A vs 7 B in SW monsoon). Influenza B peaked in November–January (north-east monsoon/winter) with 36 B vs 4 A. School-aged children (4–12 years) accounted for 71% of influenza positives. Seasonal distributions of Influenza A and B differed significantly (χ2 = 109.47; df = 2; p < 0.0001). Influenza contributes substantially to pediatric ARI in this tropical monsoon setting. Influenza A predominates in the south-west monsoon and Influenza B in the north-east monsoon/winter period. These findings support region-specific surveillance and timing of vaccination that account for local climatic cycles. Routine molecular diagnostics in pediatric ARI care will improve detection and guide public health responses.