Suboptimal vaccination and diagnostic bottlenecks fuel persistent measles transmission in The Gambia: An 11-year retrospective study

Abstract

Introduction: Despite national efforts toward measles elimination, the disease remains a significant public health challenge in The Gambia. Recurrent outbreaks are driven by suboptimal vaccination coverage, consistently falling below the 95% herd immunity threshold. This study analysed years of national surveillance data (2014–2024) to assess the epidemiology, vaccine impact, and performance of the national measles surveillance system. Methods: We conducted a retrospective, population-based cross-sectional study using the national vaccine-preventable disease (VPD) surveillance database. A total of 1,266 suspected cases were analysed. Annual incidence rates per million population were calculated, and surveillance performance was evaluated against WHO-AFRO benchmarks. Multivariate logistic regression was employed to identify independent predictors of laboratory-confirmed (IgM+) measles. Results: Of the 1,266 cases, 238 (18.8%) were laboratory-confirmed. The highest incidence was recorded in 2015 (32.8 cases per million), significantly exceeding the WHO elimination target of < 1 per million. The sensitivity of the surveillance system, measured by the non-measles febrile rash illness (NMFRI) rate, met the WHO benchmark of ≥ 2 per 100,000 population in nine out of the eleven years studied. Children aged 1–5 years represented 41.6% of confirmed cases. A strong dose-dependent protective effect was observed: receipt of ≥ 2 doses was associated with an 88.8% reduction in the odds of infection (aOR: 0.11; 95%CI: 0.06–0.20; p < 0.001). Geographically, Western Region 1 (WR1) and Western Region 2 (WR2) were identified as the primary epicentres. While the system met targets for notification (89%) and investigation (97%), sample transportation timeliness was critically low in the Upper River Region (46%). Conclusion: The Gambian measles surveillance system is sensitive and timely in detection; however, persistent transmission is associated with an immunity gap among young children in urban hotspots. The disparity in sample transportation logistics for remote regions like the Upper River Region poses a risk of delayed outbreak confirmation. We recommend that the Ministry of Health prioritise “mop-up” vaccination campaigns and supplementary immunisation activities (SIAs) specifically in Western Regions 1 and 2 to address urban transmission. Furthermore, a dedicated logistics framework must be established to improve sample transport timeliness in the Upper River Region to meet the national 5-day laboratory receipt benchmark.