Interventions to improve vaccination coverage among children aged 12–23 months in urban slum areas of Asia: a systematic review and meta-analysis

Background Low vaccination coverage among children in slum areas is a global concern, particularly in Asian countries. Despite several reported studies, systematic reviews are lacking. This systematic review aimed to identify interventions that effectively improve vaccination coverage among children aged 12–23 months in slum areas in Asia. Methods We searched the Embase, PubMed, and Cochrane databases using specific search terms for articles published between 2010 and January 2026. Two investigators independently extracted the data and assessed the methodological quality of the identified studies. Biases in non-randomized and randomized studies were assessed using the Risk of Bias In Non-randomized Studies of Interventions and Risk of Bias 2 tools, respectively. The quality of the evidence was assessed using the Grading of Recommendation, Assessment, Development, and Evaluations tool. Heterogeneity was assessed using I 2 , and subgroup analysis was performed when substantial heterogeneity was identified. The effect measure was estimated using odds ratios (ORs) and 95% confidence intervals (CIs). Results The nine included studies ranged from quasi-experimental to randomized controlled studies (RCTs) conducted in Bangladesh, India, Indonesia, and Pakistan, involving 37,405 children. The included studies demonstrated a moderate to high or serious level of bias. The full vaccination coverage probably improved in only one RCT (OR 6.42, 95% CI 4.63 to 8.90) and from three quasi-experimental studies (OR 9.68, 95% CI 3.91 to 23.95) with high heterogeneity. DPT3 vaccine coverage significantly improved in two RCTs (OR 2.68, 95% CI 2.17–3.31; 1,872 participants; I²=0%), whereas quasi-experimental studies showed uncertain effects with high heterogeneity (OR 3.58, 95% CI 2.99–4.28; 31,817 participants; I² = 78%). Compared to the control groups, single or multicomponent strategies significantly increased vaccination coverage. However, the certainty of the evidence was moderate in RCTs and very low in quasi-experimental studies owing to methodological limitations. Conclusion Owing to the methodological biases of the included studies and the limited sustainability of the implemented interventions, further studies are needed to improve vaccination coverage in children aged 12–23 months, particularly in slum areas of Asia.