Seasonality and Clinical Characteristics of Influenza-Like Illness and Severe Acute Respiratory Infection in Children Under Five Years in Cox’s Bazar, Bangladesh (2021–2023)

SUMMARY Introduction Acute respiratory infections (ARIs) such as influenza-like illness (ILI) and severe acute respiratory infection (SARI) are leading causes of morbidity among under-five children in Bangladesh. In Cox's Bazar, the ARI burden is compounded by high population density and seasonal monsoons; yet under five specific evidence on virus seasonality and clinical features remains limited. This study aimed to assess the seasonality and clinical characteristics of influenza-like illness (ILI) and severe acute respiratory infection (SARI) among children under five years of age in Cox’s Bazar, Bangladesh, between 2021 and 2023, with additional analyses to support interpretation of detection findings. Methods Prospective hospitalized based surveillance was carried out from January 2021 up to December 2023 at the District Head Quarter Hospital Cox’s Bazar. Children < 5 years old with ILI (fever ≥38°C and a cough, symptom onset ≤10 days) or SARI (ILI with hospitalization or severe manifestations) were recruited. Nasopharyngeal and throat swabs were processed for testing by a multiplex real-time reverse-transcriptase polymerase chain reaction (RT-PCR) for influenza A/B (H3N2/Victoria), SARS-CoV-2, and respiratory syncytial virus (RSV). Demographic, clinical and geographic information was recorded. Analyses included descriptive statistics, chi-square tests, and multivariable logistic regression. Results Among 968 children (median age 9 months; 60.7% ILI, 39.3% SARI), pathogens were detected in 12.4% (95% CI: 10·4–14·7%). Among the limited pathogens tested, Influenza comprised most positive cases (65%); A(H3N2) was identified in 5.0%, and B(Victoria) in 3.1%. SARS-CoV-2 and RSV were detected only occasionally (2.5% and 1.9%, respectively). The monsoon season (June-September) was the peak detection period, with A(H3N2) being dominant in June and July, and B(Victoria) dominating from August to September. Positivity was higher among SARI cases (25.8%) than ILI cases (16.4%) in the months of peak activity. Clinical severity indicators associated with detection were SARI (aOR 3.42, 95% CI: 2.28–5.13), breathlessness (aOR 2.87, 95% CI: 1.84–4.48), age ≥6 months to <2 years (aOR 1.92, 95% CI:1.21–3.05) and fever ≥101·5°F (aOR =1·68, 96%CI:100-276). The highest positivity rate observed was 24.0%. Most (87.6%) were negative for the tested pathogens. Conclusion Among the viruses tested, Influenza contributes to seasonal under five ARIs in Cox's Bazar, but the large negative proportion indicates that most cases were caused by pathogens not included in the limited testing panel (e.g., rhinovirus, adenovirus, parainfluenza, or bacteria). Markers of severity and age are associated with influenza positivity . Expanded multiplex testing and surveillance are required to inform vaccination and interventions in this high burden setting.