In the decades since they were introduced in 1997, Medicare Advantage plans have shifted the landscape of healthcare for American seniors. Heavily marketed by a few large insurance companies, Medicare Advantage plans now make up 54% of the nation’s Medicare marketplace, according to health-policy group KFF . Some 76% of Medicare Advantage plans that carry prescription-drug coverage are zero-premium, KFF says , meaning enrollees pay nothing upfront except for premiums associated with Medicare Part B (current ...