Background: We sought to characterize how the term "missed opportunities" is reported in the literature in the context of immunization rates and to assess how missed opportunities can be operationalized. Methods: Peer-reviewed literature searches were conducted in April - May, 2015, to answer: "What methods research studies used to operationalize missed opportunities to vaccinate?" A meta-narrative review methodology was used. Results: Seven studies met inclusion criteria. The methodologies for quantifying missed opportunities fell into two general categories based on: 1. the number of healthcare encounters per patient without appropriate vaccination services, defined as a number of visits per patient with no vaccination related services (Missed opportunities per patient); 2. vaccination status as "non-vaccinated" among a group of patients who had a healthcare encounter where the vaccination should/could have had happened (Missed opportunities per population). Conclusions: Our study provided an initial overview of the methods reported in the literature, and concluded that the quantifiable missed opportunity holds promise as a measurable outcome (variable) for research and quality improvement projects aimed to increase adult immunization recommendation and uptake in primary care. [ABSTRACT FROM AUTHOR]