Types, Subtypes and Positivity Rates of Seasonal Influenza in Uganda, 2019–2023

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Abstract Background Seasonal influenza, commonly known as flu, is an acute respiratory, highly contagious illness caused by influenza viruses. A clear understanding of influenza seasonality is crucial for guiding prevention and treatment strategies, including decisions on vaccination timing to prevent outbreaks. While well documented in temperate regions, data on influenza epidemiology in tropical areas, particularly sub-Saharan Africa, remain limited. We described the types, subtypes and positivity rate of seasonal influenza in Uganda during 2019–2023. Methods This study used descriptive statistics to understand the characteristics of the data, and then employed multivariable logistic regression to identify factors independently associated with a positive influenza test result. The regression analysis examined the impact of sex, sample type, time of year, and year on influenza positivity, quantifying these associations with odds ratios, confidence intervals, and p-values to assess their statistical significance. Results Of 18035 samples, 938 (5.2%) tested positive for influenza, with a higher prevalence in Influenza-Like Illness (ILI) cases (5.7%) compared to Severe Acute Respiratory Infection (SARI) cases (3.7%). Influenza A was the predominant type (69.7%), followed by influenza B (29.7%, all B. Victoria), with rare co-infections. Among influenza A subtypes, A/H3N2 was most common (65.1%), followed by A/H1N1pdm09 (33.5%). Multivariable logistic regression revealed that the male gender, the month of March, and decreasing year were associated with increased influenza positivity, while SARI sampling and other months showed reduced positivity. Conclusion The seasonal influenza positivity rates from 2019 to 2023 and the predominance of Influenza A and H3N2 highlight the need for sustained surveillance in Uganda. Given Influenza A’s high genetic variability and potential for novel strain emergence, monitoring circulating strains, informing vaccine development, and implementing targeted interventions for high-risk groups and regions are critical to controlling and preventing outbreaks.

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