This study highlights the critical role of adult immunisation programmes in addressing global health challenges and easing the pressures on healthcare systems and society. By analysing the benefits of vaccines across ten countries and four specific diseases, the report provides compelling evidence that adult immunisation not only improves individual health but also offers significant economic returns. For every dollar spent, up to 19 dollars are returned in societal benefits, translating to billions in net monetary gains. Despite the clear advantages, gaps in evidence and inconsistent access to vaccines persist, risking suboptimal policy decisions. The study advocates for a prevention-first approach, robust funding, and expanded research to fully realise the value of adult immunisation.