Strategies, interventions, and uptake of catch-up vaccination among adolescent and adult migrants, refugees, and internally displaced persons (IDPs) in low- and middle-income countries (LMICs): A systematic review

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BACKGROUND: Catch-up vaccination helps close immunity gaps among migrants, refugees and internally displaced people (IDPs) in low- and middle-income countries (LMICs). Despite immunisation life-course policies and global guidelines promoting catch-up vaccination of arriving migrants, vaccination strategies for adolescent and adult populations are poorly described. We synthesised evidence on catch-up vaccination strategies and interventions, delivery platforms, uptake and coverage, and contextual barriers and enablers in LMICs. METHODS: We searched Embase, Medline, PsycINFO, Global Health, Web of Science and grey literature sources (including websites of international and national public health organisations and agencies) for primary studies and reports on catch-up vaccination strategies and interventions, delivery platforms, uptake and coverage, and contextual barriers and enablers targeting adolescents (9-18 years) and, or adults (≥19 years) in migrants (foreign-born, including refugees) and internally displaced people (IDPs; displaced within national borders) across 136 LMICs, (from January 1st 2000 to February 1st 2025; all languages). Study quality was accessed using ROBINS-I, CASP, AACODS and, AGREE II tools. RESULTS: Thirty-seven records met the inclusion criteria (13 peer-reviewed, 24 grey literature), reporting catch-up vaccination activities across 16 LMICs. Most studies were conducted in Uganda (n = 6), Bangladesh (n = 4), Lebanon (n = 3), and Kenya (n = 3). Interventions reached ≥48,000 migrants, refugees, and IDPs (primarily Rohingya refugees in Bangladesh during COVID-19 catch-up campaigns). Populations targeted included mostly refugees (n = 16 studies; 43.2%), general migrants (n = 14; 37.8%), and IDPs (n = 5; 13.5%), with a smaller number involving mixed or other migrant groups (n = 4; 10.8%). The most frequently delivered vaccines were measles-rubella (n = 12; 32.4%), COVID-19 primary-series catch-up (n = 9; 24.3%), HPV (n = 6; 16.2%), polio OPV/IPV (n = 5; 13.5%), and Hepatitis B (n = 3; 8.1%). Catch-up vaccine delivery most commonly occurred through primary care via opportunistic offers (n = 11) and mobile/outreach delivery (n = 11), with additional implementation in fixed posts in camps/settlements (n = 7), supplemental immunisation activities (SIAs) (n = 6), school-linked delivery (n = 5), and hospital/outpatient opportunistic vaccination (n = 4). High uptake (≥85%) was reported where access barriers were minimised (e.g., walk-in availability, extended hours) was paired with community or peer engagement and simple recall systems (SMS or e-booking). Reported barriers included documentation/entitlement checks, language barriers, and fragmented or non-interoperable vaccination records. CONCLUSIONS: Migrants remain at risk of under-immunisation, and greater emphasis must be placed on promotion of vaccination across the life-course for missed vaccines, doses, and boosters. Strengthening catch-up vaccination in adolescents and adults, and improving migration-disaggregated data and delivery systems, are urgently needed.

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