In the United States, annual enrollment (also known as open enrollment or open season) is a period of time, usually but not always occurring once per year, when employees of companies and organizations may make changes to their elected fringe benefit options, such as health insurance. The term also applies to the annual period during which individuals may buy individual health insurance plans through the online, state-based health insurance exchanges established by the Patient Protection and Affordable Care Act. Open enrollment is also prominent in Medicare, where almost 50 million enrollees can choose to stay in original Medicare, or join or change plans within the Medicare Advantage and Medicare Part D Prescription Drug programs for the coming calendar year. Individuals usually can make changes to their health insurance or fringe benefits only during the open enrollment period or when they have experienced a specific qualifying event.