Exploring spillovers of vaccines: A practical application of the Expanded Impact Inventory.

Including all relevant costs and effects in economic evaluations can be challenging, especially when spillovers in other individuals or sectors are likely. Previous literature has highlighted that the combination of relevant spillovers is evaluation and context specific. This emphasises the importance of identifying (the evidence on) relevant spillovers systematically. Spillovers are defined here as "all impacts from an intervention on all parties or entities other than the users of the intervention under evaluation". In this study, we evaluate the feasibility of applying the recently introduced Expanded Impact Inventory to identify (evidence on) potentially relevant spillovers. If feasible, this approach can help to determine the analytical scope of health economic evaluations and to identify the available knowledge (gaps). As an illustration, we use two case studies: vaccination against acute respiratory diseases and herpes zoster. Using the inventory to guide a rapid review of economic studies in these disease areas, we found that commonly considered spillovers are patient healthcare and productivity costs-in so far these are borne by others than the patient-, as well as caregiving time costs. Spillover impacts on health, broader quality of life, and productivity of others, especially caregivers, were often highlighted as important, though evidence remains limited. This application demonstrates that using the Expanded Impact Inventory to identify (evidence on) potentially relevant spillovers and delineate the scope of economic evaluations is feasible. Using this framework across various interventions and disease areas can promote the systematic consideration of broader impacts in economic evaluations, strengthening the evidence base for spillovers and improving the comprehensiveness and comparability of economic evaluations.