Note: Single-source report; awaiting corroboration.
The Big Catch-Up initiative, launched during World Immunization Week 2023, has administered over 100 million vaccine doses to an estimated 18.3 million children aged 1 to 5 across 36 countries, addressing declines in vaccination coverage caused by the COVID-19 pandemic. Of these children, about 12.3 million were zero-dose—having never received any vaccine—and 15 million had not been vaccinated against measles. The campaign also delivered 23 million doses of inactivated polio vaccine (IPV) to under- or un-vaccinated children, supporting global polio eradication efforts. The program concluded on 31 March 2026, with projections indicating it will meet its target of reaching at least 21 million un- and under-immunized children.
Despite these achievements, agencies involved caution that catch-up vaccinations, while critical for closing immunity gaps, are not a substitute for expanding robust routine immunization systems. The initiative prioritized vaccine equity, focusing on children in fragile, conflict-affected, or underserved communities who often miss essential vaccinations and may be overlooked as they age. The 36 participating countries account for 60% of the global zero-dose child population, a figure worsened by pandemic-related immunization disruptions. For the first time, the initiative leveraged routine immunization systems to reach older children whose vaccinations had been delayed or missed.