“While health policy observers are mainly focused on the number of people enrolled in the new federal and state marketplaces, fewer are keeping a close eye on fairly big changes in the estimates and projections for enrollment in Medicare Advantage plans. The number of Medicare beneficiaries in Medicare private plans reached an all-time high this year of nearly 16 million beneficiaries, 6.3 million higher than the Congressional Budget Office (CBO) had projected in 2010 soon after the Affordable Care Act (ACA) was enacted (Figure 1). The CBO now projects Medicare Advantage enrollment will reach 22 million beneficiaries by 2020, more than double the number projected shortly after the ACA was enacted.”