For millions of children living in the world’s most fragile regions, the local clinic had become a place of memory rather than a point of care. During the height of the COVID-19 pandemic, health systems buckled, lockdowns restricted movement, and routine childhood immunizations—the bedrock of global pediatric health—fell precipitously. For a cohort of children born into this chaos, the window for essential first-year vaccines didn’t just close; it vanished.
The Big Catch-Up (BCU), a massive multi-year effort launched during World Immunization Week 2023, was designed to find these “invisible” children. Now, as the initiative concludes, the results signal a significant victory for global health. According to Gavi, the Vaccine Alliance, the World Health Organization (WHO), and UNICEF, the drive delivered over 100 million doses of life-saving vaccines to an estimated 18.3 million children across 36 countries.
As a physician, I have seen how the absence of a single vaccine can alter the trajectory of a child’s life. The BCU specifically targeted children aged 1 to 5 who had missed their primary series—those who should have been protected before their first birthday but remained vulnerable. By focusing on this older cohort, the initiative successfully narrowed critical immunity gaps that had widened during the pandemic, reaching 12.3 million “zero-dose” children—those who had not received a single vaccine shot in their lives.
While the program is on track to meet its ultimate target of reaching 21 million children, the success of the catch-up drive highlights a sobering reality: the global routine immunization system remains dangerously fragile. While we can “catch up” with a targeted surge of resources, the ongoing failure to reach infants in their first year of life continues to leave millions at risk.
Closing the “Zero-Dose” Equity Gap
The term “zero-dose” is more than a clinical metric; it is a proxy for extreme marginalization. Children who receive no vaccines typically live in conflict-affected zones, remote rural areas, or underserved urban slums. They are the children the system misses first and forgets longest.
The 36 countries participating in the BCU, primarily across Africa and Asia, represent 60% of all zero-dose children worldwide. To reach them, the initiative had to move beyond the traditional infant-centric model of immunization. For the first time, routine systems were systematically leveraged to screen and vaccinate children up to age five, requiring updates to national eligibility policies and intensive training for frontline health workers.
| Metric | BCU Achievement (Estimated) |
|---|---|
| Total Vaccine Doses Delivered | 100 Million+ |
| Children Reached (Ages 1-5) | 18.3 Million |
| Zero-Dose Children Reached | 12.3 Million |
| Children Receiving First Measles Dose | 15 Million |
| Inactivated Polio Vaccine (IPV) Doses | 23 Million |
The impact was most pronounced in countries where health infrastructure had been severely compromised. In Ethiopia, more than 2.5 million previously zero-dose children received their first dose of DTP (diphtheria, tetanus, and pertussis), alongside nearly 5 million doses of IPV and 4 million measles vaccines. Similarly, Nigeria reached 2 million zero-dose children with DTP1 and administered 3.4 million IPV doses.
Twelve countries—including Burkina Faso, Kenya, Pakistan, and Zambia—reported reaching more than 60% of all zero-dose children under five who had previously missed their DTP1 shot. This suggests that with sufficient political will and funding, even the most “hard-to-reach” populations are reachable.
The Persistence of Routine Failure
Despite these milestones, the global health community is issuing a stark warning: catch-up campaigns are a bandage, not a cure. The BCU served as a gap-filling measure, but it cannot replace the sustainability of a robust routine immunization program.
The data from 2024 reveals a troubling trend. While the BCU reached 12.3 million older zero-dose children, an estimated 14.3 million infants under the age of one failed to receive a single vaccine through routine services that same year. This means that as quickly as we close the gap for older children, a new generation of zero-dose infants is being created.
The consequences of these chronic gaps are already manifesting in the form of resurgence. Measles, one of the most contagious diseases known to medicine, is seeing a global spike. In 2024, approximately 11 million cases were reported, and the number of countries experiencing large-scale outbreaks has nearly tripled since 2021. This surge is driven by a lethal combination of missed routine vaccinations and a decline in vaccine confidence in previously high-coverage areas.
“Vaccinations save lives,” UNICEF Executive Director Catherine Russell noted, emphasizing that the gains made through the BCU must be sustained through long-term investment. Without a shift toward reliable, everyday immunization systems, the world remains vulnerable to preventable epidemics.
Building a Resilient Future
The legacy of the Big Catch-Up is not just the 100 million doses delivered, but the systemic improvements left behind. The initiative catalyzed new ways to identify and monitor missed children, training health workers to screen for vaccination status as a standard part of routine care. By engaging civil society and local communities, the BCU helped rebuild the trust that had eroded during the pandemic.
As we look toward the next decade, the focus is shifting toward the Immunisation Agenda 2030 (IA2030) and Gavi’s 2026–2030 strategy (Gavi 6.0). The priority remains equity—ensuring that a child’s geography or their parents’ socioeconomic status does not determine whether they survive a preventable disease.
Maintaining this momentum will require more than just international aid; it will require expanded domestic investments from governments to ensure that the “catch-up” phase becomes a permanent state of “coverage.”
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or vaccination schedule.
The global community will next convene for World Immunization Week from April 24–30, 2026, under the campaign “For every generation, vaccines work.” This event will serve as a critical checkpoint to evaluate the transition from the BCU’s emergency catch-up phase to the long-term goals of the Gavi 6.0 strategy.
Do you believe global health priorities are shifting enough to prevent the next outbreak? Share your thoughts in the comments or share this story with your network.
