Background and objective:
Undocumented migrants are among the most vulnerable in the COVID-19 pandemic. The study objective is to identify lessons learnt to strengthen migrant-sensitive health services and systems.

Methods:
Review of empirical observations by clinicians and public health practitioners, peer-reviewed journals, and grey literature analyzed through country case studies (Italy, Switzerland, France, US) on the Governance, Service Delivery, and Information building blocks of Health Systems.

Results:
Governance: Some countries expanded COVID-19 health services and alleviated immigration concerns, but implementation varied at sub-national level due to lack of specific migrant-sensitive provisions in policies, plans, and programs. Service delivery: Implementing outreach, engaging migrant communities, and integrating third sector actors are key elements to improve accessibility and uptake of health services including vaccination. Yet, these mostly operated through fixed delivery strategy, while targeted outreach through mobile clinics and outposts is more effective for people on the move. Socio-culturally sensitive information for risk communication was not developed, translated, and disseminated through the most used sources such as community networks. Authorities did not engage migrant communities and third sector actors as key partners with capacity and access. Information: Information about undocumented migrants’ health is mostly based on ad hoc surveys at selected health facilities, while there is a lack of systematic monitoring and evaluation with collection and analysis of migrant-specific data.

Conclusion:
Migrant-sensitive health services including vaccination are essential for public health. Yet, _ad hoc_ approaches resulted in low and inequitable coverage, besides lacking information for monitoring and evaluation. Health Systems can be strengthened by: (1) learning from the COVID-19 pandemic response, (2) evaluating sustainable policy and financing changes, (3) sustaining multistakeholder partnerships, (4) expanding community engagement, (5) strengthening the health workforce, (6) developing systematic monitoring and evaluation of undocumented_ _migrants’ data, (7) consider migration, including undocumented status, as a social determinant of health_._