Establishing a Seasonal Severity Assessment for Respiratory Illness- and Influenza-Associated Hospitalizations in...

In Kenya, influenza and other respiratory viruses circulate year-round. The Ministry of Health monitors severe acute respiratory infections (SARI), including influenza-associated hospitalizations, through a sentinel surveillance network. We developed intensity thresholds (ITs) to assess seasonal severity based on respiratory illness- and influenza-associated hospitalizations.

Nine surveillance sites contributed data from January 2019 through December 2025 (with rolling start dates). Weekly all-cause admissions, respiratory illnesses, and influenza testing in pediatric (0-13 years) and adult (≥ 14 years) medical wards were collected using retrospective record review (prior to June 2023) and SARI surveillance platform reporting (June 2023 onwards). Four sites had sufficient historical data to calculate ITs for the 2024 assessment; eight sites were included for 2025. We used the moving epidemic method to calculate annual and biannual ITs for moderate (50th percentile; IT₅₀), high (IT₉₀), and very high (IT₉₈) severity for the percentage of hospitalizations for respiratory illness and influenza, overall and by age group.

During 2024 and 2025, respiratory illness-associated hospitalizations remained below the IT₅₀ for most of the year across age groups, with 1-2 weeks above the IT₅₀. Influenza-associated hospitalizations also remained below the annual IT₅₀ in both years but exceeded the biannual IT₅₀ for several weeks at each wave's peak.

In 2024 and 2025, respiratory illness-associated and influenza-associated hospitalization severity was low to moderate. This approach can guide public health activities in Kenya, contribute to global influenza surveillance, and provide a framework to assess influenza severity in countries without clear Northern or Southern Hemisphere seasonality.