Vaccination timeliness is a key measure of immunisation system performance: A call to address priority research, policy and practice issues

• Vaccination coverage is widely used to assess immunisation programme delivery and performance. • Vaccination timeliness provides nuanced insights into hidden vulnerabilities in immunisation programmes, particularly for highly contagious diseases, yet it remains underexamined and under-utilised in research and policy. • Multiple gaps persist including inconsistent definitions, limited fine-scale geospatial analyses, and how data missingness is handled in timeliness analyses. • Building consensus on the definition and measurement of vaccination timeliness is essential for cross-country comparability and for integrating timeliness indicators into programme monitoring.