The research, led by Sheikh Abdulhafed Al-Shoteri of the University of Aden and the Yemen National Public Health Institute together with colleagues from Princeton University and Yemeni institutions, offers one of the most detailed portraits yet of how a time-limited catch-up campaign performs inside a fragile and conflict-affected setting. The team triangulated five data streams collected in southern Yemen in 2025: administrative immunization records, forty key informant interviews with program managers and health officials, twelve focus group discussions with caregivers and community members, a survey of 242 vaccinators, and direct observation of forty vaccination sessions. Quantitative data were analyzed descriptively while qualitative material was examined thematically, allowing the researchers to cross-check what program documents claimed against what health workers and families actually experienced on the ground.

The technical picture that emerges is one of systemic breakdown rather than isolated failure. Yemen’s routine immunization infrastructure has been decimated by years of war, producing a large cohort of zero-dose children and fueling recurrent outbreaks of measles, diphtheria, pertussis and polio, including circulating vaccine-derived poliovirus type 2. The COVID-19 pandemic then widened already severe coverage gaps by disrupting service delivery and diverting scarce resources. When the Big Catch-Up finally launched in southern Yemen in September 2024, it did so amid these pre-existing system-level failures. Updated tools and standard operating procedures, the study found, arrived only months after vaccination activities had already begun, leaving frontline teams to improvise without current guidance during the campaign’s most critical early phase.

Microplanning, the granular process by which health authorities map target populations and allocate vaccinators to specific communities, was built on a national census dating from 2004. Two decades of mass displacement had rendered those population estimates profoundly inaccurate, meaning planners were chasing targets anchored to a demographic landscape that no longer existed. In a country where millions of people have been internally displaced, families move repeatedly across governorate lines, and entire villages may be abandoned or newly settled, a census-based denominator systematically miscounts the very children the campaign was designed to find. The consequence was a planning architecture that could not reliably identify where zero-dose children lived, how many there were, or how to reach them.

Human resource constraints compounded the problem. Training for vaccinators was brief, and field supervision was consistently weak across the districts studied, according to the vaccinator survey and observed sessions. At the same time, concurrent outbreak response operations for measles and polio pulled the same limited workforce away from catch-up activities, forcing an impossible competition for staff, vehicles, cold-chain capacity and attention. Demand-generation activities, the community outreach and communication work that persuades hesitant families to bring children forward, were chronically underfunded. The study found insufficient attention to social and behavior change programming, which created additional barriers to uptake precisely where vaccine confidence was already fragile.

Caregivers and community members interviewed for the study described a web of practical and social obstacles, particularly for older children aged 24 to 59 months, who were reached at consistently lower rates than younger infants across all antigens and geographies. Distance to vaccination sites and the cost of transport weighed heavily in a country where fuel prices and road insecurity make travel expensive. Restrictions on some women’s mobility, tied to customs requiring a male escort known as a mahram, limited the ability of mothers to bring children to health facilities without a male relative available to accompany them. Rumors and fear of adverse events following immunization circulated in communities, and with underfunded communication channels there was little structured effort to counter misinformation before it took root.

The administrative reach data reveal striking variation by vaccine type, dose and geography. Reach for the third dose of pentavalent vaccine, at 3.7 percent nationally against antigen-specific targets, was less than half the figure for the first dose, a pattern consistent with the well-documented difficulty of completing multi-dose schedules in settings where families must return repeatedly to fixed sites. The consistently lower performance among children aged 24 to 59 months is epidemiologically significant because this older cohort carries much of the accumulated susceptibility driving Yemen’s measles and pertussis outbreaks. Children who missed their primary series as infants remain susceptible through early childhood, and campaigns that successfully reach infants but fail to mobilize older children leave the outbreak reservoir largely intact.

The authors’ central conclusion is sobering for global health policy: external financing and high-level policy commitment, however generous, may be insufficient to deliver equitable catch-up vaccination when implementation depends on a fragile, conflict-affected health system. The same weaknesses that have long undermined routine immunization in Yemen, from broken supply chains and absent supervision to inaccurate population data and eroded community trust, also govern the fate of emergency campaigns layered on top of them. A time-bound initiative parachuted into this environment inherits its constraints rather than escaping them. The study suggests that catch-up campaigns cannot substitute for sustained investment in primary health care, meaningful community engagement, improved population estimates and financing models that extend beyond short donor project cycles.

The findings carry implications well beyond Yemen. Fragile and conflict-affected settings now account for a growing share of the world’s zero-dose children, and the global immunization community has increasingly turned to campaign-style approaches to close pandemic-era gaps. The Yemen case demonstrates that in such settings the binding constraint is rarely vaccine supply or global funding alone; it is the operational capacity of the health system itself, including its ability to plan against real populations, supervise and support its frontline workers, communicate credibly with communities and maintain services between campaigns. Where those capacities are absent, administrative targets may be missed by an order of magnitude, and the children at greatest risk, the oldest, the most displaced and the most isolated, are systematically the last reached.

For Yemen’s Ministry of Public Health and Population and its international partners, the study points toward concrete priorities: updating population denominators to reflect displacement realities, front-loading guidance and training before activities begin, protecting immunization staff from competing outbreak demands, and funding sustained social and behavior change work rather than treating communication as an afterthought. More fundamentally, the authors argue, the international community must recognize that recovering pandemic-era immunization losses in fragile states is not a campaign problem but a health system problem, one that requires financing horizons measured in years rather than project cycles. Until the underlying architecture of primary care is rebuilt, catch-up initiatives in Yemen and similar settings will continue to measure not what the world hopes to achieve for forgotten children, but how little a broken system can deliver.

Subject of Research: Implementation of the Big Catch-Up vaccination initiative in conflict-affected Yemen and the role of fragile routine immunization systems

Article Title: Catching up in crisis: implementation of the big Catch-Up initiative in Yemen within a context of fragile routine immunization systems

Article References: Al-Shoteri, S. A., Sharkey, A., Ghouth, A. S. B., Ba-Saddik, I. A. M., AlNoban, M. S., Alwaleedi, A. A., Saleh, S. A., & Akil, N. R. H. (2026). Catching up in crisis: implementation of the big Catch-Up initiative in Yemen within a context of fragile routine immunization systems. International Journal for Equity in Health. https://doi.org/10.1186/s12939-026-03029-0

Image Credits: AI Generated

DOI: 10.1186/s12939-026-03029-0

Keywords: Big Catch-Up, Yemen, zero-dose children, routine immunization, fragile and conflict-affected settings, vaccination coverage, health systems strengthening, measles, pentavalent vaccine, immunization equity, COVID-19, primary health care