Background 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 such declines, including reduced health care seeking behaviour, lockdowns, and overwhelmed health systems. Methods We investigate determinants of RI resilience during COVID 19 at a national level for 119 countries from 2020 to 2022, using publicly available data on pre pandemic immunisation programme performance, health workforce capacity, health systems strength, health financing, global health security preparedness, COVID 19 burden, COVID-19 containment, economic, and health policy responses, population mobility changes, and country wealth. We employ a mixed methods approach: stepwise linear regression based on a causal inference framework, and Random Forest regression to identify potential nonlinear interactions and collinear effects. Results We provide evidence that stronger pre-pandemic immunisation programmes and more health workers, once above minimum thresholds (about 83% Diphtheria Tetanus Pertussis third dose coverage and 60 health workers per 10,000 population), are associated with improved RI resilience. Random Forest analysis suggests health financing and health system strength impact RI resilience. COVID 19 vaccinations and pandemic policies were not associated with RI coverage changes, implying, reassuring, these acute responses did not interrupt routine services. In addition to these findings, a large fraction of variation in pandemic RI resilience remains unexplained, highlighting the need for further research on RI performance determinants. 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. Reassuringly, we do not find evidence that COVID 19 vaccination campaigns nor pandemic containment policies impacted RI performance, counter to qualitative survey indications. We encourage continued efforts to identify RI disruption determinants to inform the evidence base for public health practitioners globally.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: - BE is currently employed by Gavi, the Vaccine Alliance. All research contained in this manuscript was conducted during a doctorate qualification, outside and independent of employment. Neither facilities, data, nor any other forms of input from Gavi, were used in this study. The research and manuscript are independent of Gavi, and the findings have not been discussed, reviewed, or endorsed by the Gavi Secretariat, or any Alliance members. - TJ acts as a consultant for the World Bank Group, the World Health Organisation, and Geomatys. This work is unrelated to the present paper and none of these organization had any impact on the study presented.

BE is a PhD student at the Institute of Global Health, Faculty of Medicine, University of Geneva, under supervision from LK, and received no funding. OK was supported by the Swiss National Science Foundation Grant [grant number PP00P3_202660]. TJ was supported by the MRC Centre for Global Infectious Disease Analysis [grant number MR/R015600/1], jointly funded by the UK Medical Research Council (MRC) and the UK Foreign, Commonwealth & Development Office (FCDO), under the MRC/FCDO Concordat agreement and is also part of the EDCTP2 programme supported by the European Union. These funders had no role in the design and conduct of this study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

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