This review explores and positions the value of serology testing to support current immunization policies and the broader policy response to the coronavirus disease 2019 (COVID-19) crisis in Europe. We applied an exploratory approach to analysing existing evidence, international recommendations, and national policies using desk research from secondary sources, document analysis, and expert information. Regional and country-level resources from five focus countries were included: France, Germany, Italy, Spain, and the United Kingdom. Seven experts in the fields of COVID-19 immunization, serology testing, seroepidemiology, and vaccine safety and effectiveness studies contributed to the review and convened in two online panel sessions. The paper includes an overview of (1) the impact of the pandemic to date, (2) testing strategies, (3) COVID-19 vaccination policies, (4) lessons on using serology testing to support immunization, (5) current policies and recommendations on the use of a serology testing strategy, and (6) implementation barriers and challenges. Finally, this paper also provides a set of knowledge-based recommendations to advance the effective and timely inclusion of serology testing and resolve impeding knowledge gaps. The recommendations herein are intended to support timely decision-making, raise awareness, guide advocacy initiatives, and inspire future studies.