Menopausal hormone therapy, migraine history, and headache severity: Results from the Women's Health Initiative Hormone Therapy clinical trials.

The burden of vaccine-preventable illness in the United States has persisted despite decades of investment in immunization infrastructure. Safe, effective vaccines now exist for a widening range of conditions, yet adult uptake rates have remained stubbornly below national target goals, and vaccine hesitancy has resisted simple, single-lever solutions. A pivotal inflection point arrived with the passage of the Affordable Care Act in 2010. The law eliminated cost-sharing for recommended vaccines and extended insurance coverage to large numbers of previously uninsured and underinsured adults, dismantling one of the most cited access barriers. The ACA redrew the policy landscape, as financial access could no longer serve as a sufficient explanation for the gap between vaccine availability and vaccine uptake. This has demonstrated that structural access, while foundational, has not been sufficient to achieve population-level immunity. The evidence has increasingly pointed toward the need for interventions that operate simultaneously at behavioral, informational, structural, and systems level. The ACA-era policy environment was not, however, uniformly favorable to adult vaccine access. Medicaid expansion became politically contested with a number of states declining or delaying expansion, and adult immunization benefits within Medicaid remained inconsistent even where expansion occurred. A 2019 national survey found that only 22 of 51 state Medicaid programs covered all 13 Advisory Committee on Immunization Practices (ACIP)-recommended adult vaccines for both fee-for-service and managed care enrollees. The COVID-19 pandemic later compounded this uneven landscape, introducing challenges including diminished trust in government, pharmaceutical, and public health institutions, alongside structural barriers to vaccination access disproportionately affecting rural and racial/ethnic minority communities, such as limited hours of operation, transportation constraints, and unequal access to online scheduling. In response to that recognition, substantial post-ACA literature has documented diverse intervention approaches, including reminder-recall systems, provider recommendations, motivational interviewing, community-based outreach, incentive programs, and mandate policies. That literature has grown unevenly, accelerating in the wake of COVID-19, and has accumulated across heterogeneous vaccine types, populations, delivery settings, and intervention modalities. A consolidated synthesis specifically anchored to the U.S. policy environment of the past 15 years has been absent. Prior systematic reviews have examined discrete segments of this terrain, such as influenza uptake among healthcare workers, HPV vaccination in young adults, and COVID-19 hesitancy, but integrative analysis spanning a large range of recommended adult vaccines has been rare. Reviews that have taken a broader view of adult immunization have either predated the ACA or drawn on non-U.S. populations, limiting their relevance to current U.S. practice and policy.