In the United States, Medicare is a national social insurance program, administered by the U.S. federal government since 1966, currently using about 30 private insurance companies across the United States. Medicare provides health insurance for Americans aged 65 and older who have worked and paid into the system. It also provides health insurance to younger people with disabilities, end stage renal disease and amyotrophic lateral sclerosis. In 2010, Medicare provided health insurance to 48 million Americans—40 million people age 65 and older and eight million younger people with disabilities. It was the primary payer for an estimated 15.3 million inpatient stays in 2011, representing 47.2 percent of total aggregate inpatient hospital costs in the United States. Medicare serves a large population of elderly and disabled individuals. On average, Medicare covers about half of the health care charges for those enrolled in Medicare. The enrollees must then cover the remaining approved charges either with supplemental insurance or with another form of out-of-pocket coverage. Out-of-pocket costs can vary depending on the amount of health care a Medicare enrollee needs.