Thursday marked the launch of a major restructuring at MassHealth that will affect the care delivery of 1.2 million people insured by the state Medicaid program, a move intended to improve overall health while containing costs.

“This program is not the first Medicaid ACO initiative in the country, but is one of the most ambitious in terms of the percentage of our Medicaid population that will be affected, the downside risk that is required of all ACOs, and the amount of detail put into its development,” said Steve Walsh, president of the Massachusetts Health and Hospital Association.

Under the new system, 17 accountable care organizations will cover between 800,000 and 850,000 Masshealth members, with the ACOs – networks of physicians, hospitals and other health providers – responsible for the total cost and quality of care for patients.

According to the Executive Office of Health and Human Services, the ACOs will be measured based on members’ satisfaction with care and will be accountable for providing preventative care, managing chronic diseases, integrating behavioral and physical care, and ensuring appropriate follow-up care after a hospitalization.

The ACOs will work with 27 “community partners” statewide to provide specialty services and care coordination for members with complex behavioral and long-term needs, according to the administration. The community partners will officially launch in June.

MassHealth members are automatically enrolled in one of the ACOs based on their primary care provider, and will be able to change their plan for any reason for 90 days. Members who opt out of the ACOs can instead choose to enroll in one of two managed care organizations, which together will cover between 150,000 and 200,000 members, or MassHealth’s primary care clinician plan, also expected to cover between 150,000 and 200,000 members.