Objectives: The adverse impact of the COVID-19 pandemic on Routine Immunisation (RI) coverage has been well-documented: most countries experienced backsliding or stagnation in coverage. Qualitative surveys indicated potential causes of declines, including reduced health care seeking behaviour, lockdowns, and overwhelmed health systems. We investigate country-level determinants of RI resilience during COVID-19 globally to inform the evidence base on maintaining robust immunisation systems in times of crises.

Study design: Ecological, secondary data analysis with multivariate modelling.

Methods: We employ two methods: stepwise linear regression based on a causal inference framework, and Random Forest regression on a dataset comprising 13 potential determinants (spanning pre-pandemic immunisation programme performance, health workforce capacity, health systems strength, financing, global health security preparedness, COVID-19 burden, COVID-19 policy responses) and covering 151 countries from 2020 to 2022.

Results: We provide evidence that stronger pre-pandemic immunisation programmes (p < 0.0001) and more health workers (p = 0.0065), once above minimum thresholds (78 % Diphtheria-Tetanus-Pertussis third-dose coverage and 58 health workers per 10,000 people), are associated with improved RI resilience. Random Forest analysis suggests health financing and health system strength impact RI resilience. Reassuringly, we do not find evidence that COVID-19 vaccination campaigns nor pandemic containment policies impacted RI - counter to qualitative survey indications.

Conclusion: Our findings underscore the role of robust immunisation programmes and sufficiently sized health workforces in mitigating RI disruption during global health crises, once above minimum thresholds. A large fraction of variation in pandemic RI resilience remained unexplained through our population-level analyses.