{ "background": "Public health surveillance systems are critical for early detection and response to outbreaks, yet longitudinal evidence of their direct impact on population-level risk reduction in sub-Saharan Africa remains scarce. Methodological challenges in attributing health outcomes to surveillance performance persist.", "purpose and objectives": "This study aims to evaluate the longitudinal performance of integrated disease surveillance and response (IDSR) systems and quantify their causal effect on reducing epidemic-prone disease risk.", "methodology": "A quasi-experimental, longitudinal design was employed, utilising panel data from sentinel surveillance sites. The primary analysis fitted a generalised estimating equation model: $logit(P(Y{it}=1)) = \\beta0 + \\beta1(Performance{it}) + \\beta2 X{it} + \\alphai + \\epsilon{it}$, where $Y{it}$ is outbreak incidence at site $i$ and time $t$, $Performance{it}$ is a composite index, and $X_{it}$ are time-varying confounders. Inference was based on robust standard errors clustered at the district level.", "findings": "Analysis indicates a significant inverse relationship between surveillance performance and outbreak incidence. A one-standard-deviation increase in the composite performance score was associated with a 22% reduction in the odds of an outbreak (OR = 0.78, 95% CI: 0.65 to 0.94).", "conclusion": "Enhanced surveillance system functionality, measured longitudinally, is a significant determinant of reduced epidemic risk. Sustained investment in core surveillance capacities yields measurable public health returns.", "recommendations": "Policy should prioritise consistent funding for surveillance core functions and data quality audits. Programme evaluations should adopt similar longitudinal, quasi-experimental designs to strengthen causal inference.", "key words": "surveillance evaluation, quasi-experiment, longitudinal study, risk reduction, health systems, East Africa", "contribution statement": "This study provides novel longitudinal, causal evidence on the effectiveness of surveillance