Strengthening catch-up vaccination delivery in routine health services for children under five in Western Kenya, 2022–2024

BACKGROUND: Catch-up vaccination can close population immunity gaps, yet integration into routine immunization services remains limited in low- and middle-income countries (LMICs). Following Kenya's 2023 catch-up vaccination policy, we evaluated a multi-strategy intervention to strengthen systematic identification, eligibility screening, and delivery of missed doses within routine health services in Siaya County. METHODS: Over 12 months, five strategies addressing healthcare worker (HCW) capacity, vaccination documentation, defaulter tracking, immunization screening, and caregiver engagement were implemented in 55 public health facilities selected through stratified random sampling. Using a pre-post design, we assessed immunization uptake, missed opportunities for vaccination (MOVs), and system readiness. Baseline (November 2022-January 2023) and endline (October-December 2024) evaluations included facility assessments, HCW interviews, caregiver exit surveys among children aged <36 months with home-based records (HBRs; n = 658), and facility immunization register abstractions (n = 1650 children aged 12-23 months). Analyses accounted for sampling design, clustering, and used generalized estimating equations for HBR data. RESULTS: MOV prevalence among children visiting facilities for any reason declined from 94.4% to 20.9% (p < 0.001). Full immunization among children aged 12-23 months increased from 44.9% to 63.6% (p < 0.001), with uptake improving across antigens. Measles-rubella first-dose uptake increased from 51.5% to 78.0%, and second-dose uptake from 18.8% to 36.9% (p < 0.001 for both). Eligibility screening expanded from 60.6% to 86.8% of facilities (p = 0.012), defaulter tracking improved from 48.5% to 91.6% (p < 0.001), and HCW knowledge and confidence increased significantly. CONCLUSION: The multi-strategy intervention was associated with fewer MOVs and improved uptake, suggesting that embedding catch-up vaccination within routine health services can strengthen immunization systems in decentralized LMIC contexts. Kenya's catch-up vaccination policy provided a framework for introducing catch-up vaccination into routine health services.