Taking stock of global immunisation coverage progress: the gains, the losses and the journey ahead.

Despite the enormous benefits of vaccination, global immunisation coverage progress has stalled and remains suboptimal in many countries. In this commentary, we review the recently published update of the World Health Organization and United Nations Children's Fund Estimates of National Immunization Coverage. We highlight trends in which, despite substantial gains made in improving immunisation coverage at the global level, there remain numerous challenges with reaching and sustaining optimal coverage. We contextualise the trends by exploring plausible supply- and demand-side root causes. Based on these, we stress the need for targeted, context-appropriate strategies for reaching and maintaining optimal immunisation coverage.