Complex sentence: In the United States, Medicare is a single-payer, national social insurance program administered by the U.S. federal government since 1966, currently using about 30--50 private insurance companies across the United States under contract for administration.
Split into simpler sentences: In the United States, Medicare is a single-payer, national social insurance program administered by the U.S. federal government since 1966. Currently using about 30--50 private insurance companies across the United States under contract for administration.