Background

Decades of conflict in Yemen have decimated routine immunization infrastructure, produced a large population of zero-dose children, and precipitated recurring outbreaks of measles, diphtheria, pertussis, and polio. The coronavirus disease 2019 (COVID-19) pandemic further widened coverage gaps. This paper examines how pre-existing system fragility shaped the implementation and outcomes of the Big Catch-Up (BCU) initiative in southern Yemen.

Methods

A mixed-methods case study was conducted in southern Yemen in 2025. Data sources included administrative immunization records, key informant interviews (n = 40), focus group discussions (n = 12), a vaccinator survey (n = 242), and observation of 40 vaccination sessions. Qualitative data were analyzed thematically; quantitative data were analyzed descriptively. Findings were triangulated across sources.

Results

By July 2025, national BCU administrative reach against nationally estimated antigen-specific targets was 7% for the first dose of pentavalent vaccine (Penta1), 3.7% for the third dose (Penta3), and 7.6% for the first dose of measles-containing vaccine (MCV1), with children aged 24–59 months reached at consistently lower rates. Implementation was launched in September 2024 in southern Yemen amid system-level failures rooted in pre-existing fragility. The administrative reach in southern Yemen shows variations in vaccine-specific type and dose and geography. Updated tools and standard operating procedures arrived months after activities commenced; microplanning relied on a 2004 census that failed to account for mass displacement; demand-generation activities were underfunded; training was brief; and field supervision was consistently weak. Concurrent outbreak responses further strained the workforce. Distance, transport cost, restrictions on some women’s mobility, rumors, and fear of adverse events were also reported as barriers, particularly for older children. Insufficient attention to social and behavior change activities created additional barriers to uptake.

Conclusions

External financing and policy commitment may be insufficient for equitable catch-up vaccination when implementation depends on a fragile, conflict-affected health system – the same weaknesses that have long undermined routine immunization in the country. Time-bound campaigns cannot substitute for sustained primary health care investment, meaningful community engagement, improved population estimates, and financing models that extend beyond donor project cycles.