Immunization, hailed as one of the most successful public health interventions in the world, has contributed to major advancements in health as well as social and economic development [1]. Vaccines help to avert more than 20 life-threatening diseases and are responsible for preventing an estimated 3.5 to 5 million deaths each year [2]. Following the introduction of the Expanded Immunization Programme by the World Health Organization (WHO) in 1974 [3], there were dramatic gains in immunization coverage worldwide, bolstered by global collaborative efforts to increase coverage and expand immunization among under-vaccinated populations.

Yet, in recent years, progress has largely stalled and, in some cases, reversed. Although these trends were becoming evident prior to the COVID-19 pandemic [4], they have been greatly exacerbated since the onset of COVID-19 and associated disruptions in 2020. Childhood immunization programmes have lost ground, with an estimated 25 million children under the age of 1 not receiving a third dose of diphtheria-tetanus-pertussis-containing vaccine (DTP3) in 2021—the highest number for more than a decade; 18 million of these children did not even receive the first dose of DTP vaccine (zero-dose children)[5]. Between 2019 and 2021, there were decreases in global coverage of the first dose of Human Papillomavirus vaccine (HPV) among girls (from 20% in 2019 to 15% in 2021)[5], and coverage decreases were reported across many other WHO-recommended vaccines, including polio, pneumococcal, rotavirus, and measles-containing vaccines [6]. Against this backdrop of slowed progress, inequalities are an increasingly highlighted concern as certain population groups remain systematically at risk of being unvaccinated or under-vaccinated. More than 60% of unvaccinated or under-vaccinated children in 2021 lived in just 10 countries (India, Nigeria, Indonesia, Ethiopia, the Philippines, the Democratic Republic of the Congo, Brazil, Pakistan, Angola, and Myanmar)[5], and unvaccinated children remain disproportionately represented in impoverished, rural or urban slum areas, and situations of conflict or fragility [7]. Meanwhile, with recent disruptions to immunization programs, inequalities have emerged or become worse in many middle-income countries that have typically had high-performing programs [8].