Mapping national and subnational zero-dose prevalence and multidimensional inequities across 77 countries

Vaccines are one of the most powerful tools in public health, acting as a shield that has saved millions of children from diseases that once killed or disabled them. For decades, global health efforts have focused on getting vaccines into as many arms as possible, and the results have been remarkable: today, nine out of ten infants worldwide receive at least one dose of a vaccine. Yet, despite this success, a stubborn group of children remains untouched by these life-saving shots. These are the "zero-dose" children, a term used by health experts to describe infants who have not received even a single dose of a routine vaccine. Being zero-dose is not just a matter of missing one appointment; it is often a sign that a family is cut off from the entire health system, facing barriers like poverty, lack of education, or living in remote areas where clinics do not reach. Understanding who these children are and where they live is critical, because if we cannot find them, we cannot protect them.

A new study brings a sharper lens to this problem, moving beyond simple national averages to reveal the hidden landscape of inequality. Researchers analyzed data from over 170,000 children aged 12 to 23 months across 77 countries, using information from large household surveys conducted between 2015 and 2024. Instead of just looking at the national percentage of unvaccinated children, they mapped these numbers down to specific regions within each country, covering more than 1,500 local administrative areas. They also looked at the lives of these children through multiple dimensions, checking factors like their mothers' education, family wealth, where they live, and whether their families had health insurance. The goal was to see if the children who missed vaccines were clustered together in specific pockets of disadvantage and to understand what specific barriers were keeping them away.

The study found that the problem is far more uneven than national numbers suggest. While some countries have very low rates of zero-dose children, others have rates that are alarmingly high. In six countries, including Guinea, Nigeria, and the Democratic Republic of the Congo, more than 30% of children had received no vaccines at all. In the most extreme cases, some local regions had rates exceeding 50%, meaning half the children in those areas were completely unreached. However, looking at the country as a whole often hides these dangerous pockets. A nation might have a moderate average, but inside its borders, there could be specific districts where vaccination has almost completely failed. The researchers found that in countries like the Democratic Republic of the Congo, Nigeria, and Angola, these high-prevalence areas were not scattered randomly but were concentrated in specific localities, creating "islands" of vulnerability that national statistics would miss.

Beyond geography, the study confirmed that zero-dose children are almost always the children of the most disadvantaged families. Across every country studied, these children were consistently found among households with lower wealth, less educated mothers, and those living in rural areas without health insurance. The researchers measured how strongly these disadvantages were linked to missing vaccines and found that in some places, the gap between the richest and poorest families was massive. For instance, in Azerbaijan, the difference in vaccination rates between the most and least advantaged groups was nearly 88 percentage points. This pattern suggests that being zero-dose is rarely an accident; it is usually the result of a family facing multiple overlapping hurdles that prevent them from accessing care.

Perhaps the most revealing part of the study was how the reasons for these gaps changed from one country to another. There is no single "fix" that works everywhere. In some nations, the biggest barrier was a mother's lack of education; in others, it was simply having no money. In a few places, the distance to a clinic or the lack of health insurance was the primary driver. In some settings, the researchers found that the factors they measured did not explain the problem at all, suggesting that other hidden issues—perhaps related to local politics, conflict, or specific cultural barriers—were at play. This means that a strategy that works in one country might fail in another. To reach the children who are left behind, health programs need to know exactly which barrier is the most important in their specific location.

The study also highlighted a complex relationship between how many children are missing vaccines and how concentrated that problem is among the poor. Some countries face a double challenge: they have a huge number of unvaccinated children, and those children are heavily concentrated among the poorest families. These nations need broad efforts to strengthen their entire health system while also targeting the poorest communities. Other countries have very few unvaccinated children overall, but the few who remain are almost entirely from the most disadvantaged groups. In these places, the solution is not to build more clinics for everyone, but to find those specific, hard-to-reach families and ensure they are not forgotten.

Ultimately, this research shows that the fight for vaccine equity is not a single battle but many different ones, each with its own terrain. By mapping the problem down to the local level and understanding the specific mix of barriers in each place, health leaders can stop guessing and start targeting. The data provides a clear roadmap: to reach the zero-dose children, we must look past the national averages, find the specific pockets where children are being missed, and tailor the solution to the unique reasons they are being left behind. Only by understanding these local realities can the global goal of leaving no child behind become a reality.