Investigation of an influenza A(H1N1)pdm09 outbreak in a high-mobility border district, southwestern Uganda.

OBJECTIVES

We investigated to confirm an influenza A(H1N1)pdm09 outbreak, determine its magnitude, identify factors associated with severe illness, and inform public health response measures in Kisoro District, a high-mobility cross-border district in Uganda.

METHODS

We investigated suspected cases from December 1, 2024 to April 4, 2025 using record review and case interviews. Suspected cases had fever with runny nose, cough, or sore throat for ≤10 days plus ≥1 additional symptom or clinician-diagnosed respiratory infection. Severe acute respiratory infection (SARI) was defined as hospitalization or respiratory illness with difficulty breathing. Nasopharyngeal swabs were tested using reverse transcriptase polymerase chain reaction. Factors associated with SARI were assessed using multivariable logistic regression.

RESULTS

We identified 3464 suspected cases; 27% (9/33) of human samples tested positive for influenza A(H1N1)pdm09. Among suspected cases, the attack rate was highest among children aged <1 year (239/15,845; 15/1000) and those ≥60 years (179/13,365; 13/1000). School-aged children (1-17 years) accounted for 50% of all cases. SARI accounted for 6% (200/3464) of cases. SARI was associated with age 1-17 years (adjusted odds ratio (AOR) = 2.6, 95% confidence interval (CI) = 1.0-6.7) and chronic conditions (AOR = 52, 95% CI = 5-579). Urban residence (AOR = 0.21, 95% CI = 0.1-0.4) and being a pupil or student (AOR = 0.08, 95% CI = 0.02-0.17) were protective.

CONCLUSION

We confirmed a widespread influenza A(H1N1)pdm09 outbreak, with predominant transmission among school-going children and greater severity among persons with chronic conditions. Strengthening targeted surveillance, expanded respiratory testing, and school-focused health education are critical to mitigate future outbreaks, particularly in rural and high-mobility cross-border settings.