The Big Catch-Up (BCU), a historic multi-year, multi-country effort to address vaccination declines driven largely by the COVID-19 pandemic, has reached an estimated 18.3 million children aged 1 to 5 across 36 countries with more than 100 million doses of life-saving vaccines, helping to narrow critical immunity gaps, announced Gavi, the Vaccine Alliance (Gavi), WHO, and UNICEF at the start of World Immunization Week.