Why the world’s largest immunisation recovery effort highlights both the potential and the fragility of routine health systems
How does a global health system, after decades of established vaccination programmes, reach a point where a catch-up campaign is needed for more than 18 million children?
This question lies at the heart of the Big Catch-Up (BCU), a multi-country initiative that delivered more than 100 million vaccine doses to previously-missed children across 36 countries. The conclusion of the initiative that was started in 2023, was announced during World Immunisation Week in March 2026, and marks the largest coordinated immunisation recovery effort in history.
While the scale of the achievement is significant, it also exposes a deeper reality. Large-scale recovery efforts demonstrate what is possible when governments, partners, and health workers mobilise at speed and scale. At the same time, they reveal persistent weaknesses in routine health systems that allowed millions of children to be missed in the first place.
The persistence of missed children
Immunisation remains one of the most effective public health interventions, preventing millions of deaths each year. Yet in 2024 alone, an estimated 14.3 million infants globally failed to receive a single routine vaccine.
The COVID-19 pandemic did not create this gap, but it magnified existing vulnerabilities. Disruptions to service delivery, workforce shortages, supply-chain instability, and declining trust in health services combined to push millions of children out of reach. These effects were felt most acutely in fragile, conflict-affected, and underserved communities.
As a result, a growing cohort of “zero-dose” children has emerged. These are children who have never received a single vaccine and who often age beyond infancy without ever being reached by routine immunisation services.
A system under pressure
The Big Catch-Up was more than a vaccination campaign. It functioned as a real-world stress test of national health systems.
To reach children aged one to five who had been missed, countries were required to adapt rapidly. Health workers were retrained to identify older unvaccinated children, outreach strategies were expanded beyond fixed clinics, and data systems were strengthened to locate children previously absent from official records. In many settings, vaccination checks were integrated into broader child health services to improve identification and follow-through.
These adjustments highlight an important systems insight. Health outcomes depend on how well policy, workforce capacity, data systems, financing, and community engagement function together. When one component weakens, gaps in access widen quickly, often without immediate visibility, until they require emergency correction.
What the data shows
Between 2023 and March 2026, the Big Catch-Up delivered more than 100 million vaccine doses, reaching an estimated 18.3 million children. Of these, 12.3 million were zero-dose children, and 15 million had never received a measles vaccine. The initiative also delivered 23 million doses of inactivated polio vaccine, supporting global eradication efforts.
In the joint announcement marking the conclusion of the initiative, World Health Organization Director-General Dr Tedros Adhanom Ghebreyesus noted that the Big Catch-Up has helped “undo one of the pandemic’s major negative consequences” by protecting children who missed vaccinations due to disruptions in routine health services. He emphasised that the effort has also strengthened the ability of health systems to identify and reach children who are routinely missed.
Several participating countries succeeded in reaching more than 60% of previously missed children under the age of five. This demonstrates that progress is possible even in resource-constrained environments when systems are intentionally strengthened and adequately supported.
However, these gains remain fragile. Measles outbreaks are increasing globally, with approximately 11 million cases reported in 2024. This resurgence is driven largely by persistent gaps in routine immunisation coverage and serves as a reminder that catch-up efforts, while essential, cannot substitute for consistent, everyday service delivery.
Reaching children beyond infancy
An illustrative example helps to clarify this point. In a lower-income country affected by internal displacement, routine immunisation services had historically focused on infants attending fixed clinics. Many children born during periods of instability were never registered and aged beyond eligibility windows without contact with health services.
During the Big Catch-Up, health workers were trained to integrate vaccination status checks into every child health visit, regardless of age. Community outreach teams supported household mapping, while simplified data tools tracked previously unrecorded children. Within two years, thousands of older children received their first vaccines, and the revised approach was incorporated into routine service delivery.
The significance lay not only in the numbers reached, but in the institutional shift. Missed children became a routine responsibility rather than an exceptional concern.
Catch-up is necessary, but not sufficient
The central lesson of the Big Catch-Up is clear. Large-scale recovery efforts can close immunisation gaps, but they cannot replace strong routine systems.
Catch-up campaigns are inherently resource-intensive and time-bound. Their greatest value lies in restoring coverage while strengthening the systems needed for continuity. Without sustained investment in everyday service delivery, the same gaps are likely to re-emerge, particularly in settings facing rising birth cohorts, conflict, displacement, and fiscal pressure.
For health system leaders, the challenge is not whether catch-up works, but how the momentum it generates is embedded into routine practice.
What needs to happen next
Strengthening routine immunisation must remain central to primary healthcare delivery. This requires sustained investment in the health workforce, including training and supervision that require health workers to integrate vaccination status checks into every child health visit, regardless of age.
Improved data systems are equally critical. Children who are not counted are not reached, and reliable, community-linked data is essential for early identification. Building and maintaining community trust must remain a priority, alongside designing systems that are resilient enough to withstand future shocks without disrupting essential services.
From recovery to resilience
The Big Catch-Up demonstrates what is possible when global agencies, governments, and communities act with urgency and coordination. Millions of children are now better protected from preventable disease, and important system improvements have been catalysed.
Its deeper significance lies in what it reveals. Sustainable progress depends not on extraordinary campaigns, but on ordinary systems that function consistently, equitably, and at scale.
As global health priorities advance under the Immunisation Agenda 2030, the focus must shift from catching up to ensuring that no child is missed in the first place.
Leadership implications
The lessons of the Big Catch-Up extend beyond emergency recovery efforts. They point to the need for sustained leadership attention to routine immunisation systems, workforce capacity, data quality, and community trust. Translating global initiatives into durable national impact requires institutions and leaders who can move from insight to implementation, long after crisis funding and attention have faded.
Author: Lilly Seoka
Editorial Support: Dr Gustaaf