The Persistence of Medicare Advantage Spillovers in the Post-Affordable Care Act Era

Spillovers can arise in markets with multiple purchasers relying on shared producers. If producers are constrained in their ability to adjust quality and cost across purchasers, then the influence of a dominant purchaser affects the entire market. Prior studies have found such spillovers in health care, from managed care to non-managed care populations — reducing spending, utilization, and improving outcomes. Similar effects have been identified in the Medicare Advantage market as well, with studies finding declines in utilization and reductions in resource use among the Traditional Medicare population associated with increases in county-level Medicare Advantage penetration. However, no study to date has provided plausibly causal estimates of such spillovers in the post-Affordable Care Act era. Our study does so by exploiting idiosyncratic differences in payments to Medicare Advantage plans that are unrelated to traditional Medicare spending. Further controlling for health status and other potential confounders, we estimate that a one percentage point increase in county-level Medicare Advantage penetration results in a $146 (1.7%) reduction in standardized per enrollee Traditional Medicare spending. We find evidence for reductions in utilization both on the intensive and extensive margins (including reductions in the number of inpatient stays) and across many types of health care services including: home health, inpatient, and imaging, not all of which have been analyzed in prior Medicare Advantage spillover studies. Our results suggest that spillovers from Medicare Advantage to Traditional Medicare have persisted in the post-Affordable Care Act era.