Determinants of zero-dose status and a framework for institutionalizing the Big Catch-Up in fragile settings: operational evidence from the Central African Republic

In the vast landscape of global public health, few goals are as fundamental or as urgent as ensuring every child receives the basic protection of vaccines. For decades, health workers have known that the most vulnerable children—those living in the most remote villages, the most conflict-ridden zones, or the most unstable communities—are often the ones left behind. These children, who have never received even a single shot of a vaccine, are known as "zero-dose" children. They represent a critical failure in the system, a gap where disease can take hold and spread. While it is often assumed that these children are missed because their families refuse vaccines or do not believe in them, the reality in many fragile nations is far more complex. The barriers are often physical and structural: roads that disappear in the rainy season, clinics that run out of medicine, or health workers who simply cannot reach a village without a vehicle. Understanding exactly why these children remain unvaccinated is the first step toward fixing the system, not just for a single campaign, but for the long term.

In the Central African Republic, a nation grappling with chronic insecurity and displacement, a massive effort called the "Big Catch-Up" was launched to find and vaccinate these missed children. This initiative was not a one-time event but a sustained operation involving health workers, community volunteers, and international partners traveling across the country to identify children who had fallen through the cracks. Researchers later looked back at the records generated by this effort to understand the specific reasons why nearly half of the children they found had never received their first dose of a key vaccine. They analyzed data from over 800 children, piecing together a clear picture of the obstacles that prevent immunization in one of the world's most challenging environments.

The study revealed that the reasons children remained unvaccinated were far more practical than ideological. The most common barrier was not a refusal to vaccinate, but a simple lack of knowledge; in nearly one-third of the cases, caregivers did not know when or where to bring their children for shots. This was followed closely by physical barriers: many families lived in areas that were geographically inaccessible, or they were simply too far away from a vaccination site, often more than five kilometers. In many instances, the children or their parents were absent when the health team arrived, or the vaccination sessions themselves had been cancelled due to insecurity or logistical failures. The data showed that shortages of trained staff and broken supply chains also played a significant role, preventing services from reaching those who needed them most.

Perhaps the most surprising and significant finding was the role of the information system itself. In many of these fragile settings, vaccination records are kept on paper cards that can be lost, damaged, or left behind when families move. The researchers found that a lack of recorded history was a major driver of zero-dose status. In nearly 15 percent of the cases, the primary issue was that a child had actually been vaccinated, but the event was never written down or entered into a system that could track them. When looking at the broader category of information system constraints—which includes missing cards and incomplete registers—this factor accounted for over 34 percent of all operational determinants. Without a reliable record, health workers could not tell if a child was missing a shot or if they had already received it, leading to confusion and missed opportunities. This suggests that the problem is often not that the vaccine is unavailable, but that the system cannot see the child.

When the researchers looked deeper into the data, they found that the idea of vaccine refusal or mistrust was much less common than often assumed. While rumors and hesitation did exist in some communities, they accounted for a very small fraction of the missed vaccinations compared to the sheer weight of access and service delivery problems. The study explicitly showed that the children were not being excluded because their families were afraid of vaccines, but because the health system could not reach them, or because the system failed to keep track of them once they were reached. The presence of a vaccination card, which served as proof of previous care, was actually a protective factor, significantly lowering the chance that a child would be classified as zero-dose.

The implications of these findings point toward a clear path forward. To truly reduce the number of zero-dose children in fragile settings like the Central African Republic, the focus must shift from simply running vaccination campaigns to building a system that can continuously find and follow these children. This means investing in electronic records that can survive displacement and loss, training more health workers to reach remote areas, and ensuring that vaccination sessions are reliable and frequent. The "Big Catch-Up" initiative proved that it is possible to find these children, but the study concludes that making these gains permanent requires institutionalizing these efforts into everyday health services. By treating the identification of missed children as a routine part of the job, rather than a special emergency project, health systems can begin to close the gap that leaves the most vulnerable children behind.