{ "background": "Public health surveillance systems are critical for disease control, yet their methodological evaluation, particularly regarding cost-effectiveness, remains underdeveloped in many resource-limited settings. There is a pressing need for robust analytical frameworks to guide investment and optimisation of these systems.", "purpose and objectives": "This intervention study aimed to methodologically evaluate the cost-effectiveness of existing public health surveillance systems in Kenya and to develop a modelling framework for identifying the most efficient configurations for future investment.", "methodology": "We conducted a multilevel regression analysis of surveillance system performance and cost data from multiple administrative levels. Cost-effectiveness was modelled using a two-level hierarchical model: $\\text{log}(\\text{Effectiveness}{ij}) = \\beta{0j} + \\beta{1}\\text{log}(\\text{Cost}{ij}) + \\epsilon{ij}$, where $\\beta{0j} = \\gamma{00} + u{0j}$, with subscripts $i$ and $j$ denoting facilities and counties, respectively. Uncertainty was quantified using 95% confidence intervals derived from robust standard errors.", "findings": "The analysis identified significant diminishing returns on investment for syndromic surveillance systems. A 10% increase in operational expenditure was associated with only a 2.3% (95% CI: 1.7 to 2.9) increase in core performance metrics, indicating poor marginal cost-effectiveness for higher expenditure tiers. System fragmentation was a key thematic driver of inefficiency.", "conclusion": "Current surveillance architectures exhibit suboptimal cost-effectiveness, with significant inefficiencies arising from fragmented implementation and disproportionate spending on established, high-cost modalities.", "recommendations": "Resource allocation should prioritise integrated, community-based surveillance components over further investment in standalone syndromic reporting. Policymakers should adopt the multilevel modelling framework for periodic system evaluation and strategic planning.", "key words": "health surveillance, cost-effectiveness analysis, multilevel modelling, health systems, Kenya, public health", "contribution statement":