BOSTON — Nearly five months after the first COVID-19 vaccines were administered in Massachusetts, Gov. Charlie Baker said Monday the state will move toward a new phase of its immunization effort with the state on track to meet its initial goal of fully vaccinating 4.1 million people by early June. Baker said the revised vaccine strategy will see the administration start to dial back its reliance on mass vaccination sites and increase vaccine distribution to regional vaccine sites, mobile clinics and primary care providers to improve access to harder to reach populations. Four of high-volume mass vaccination sites - Gillette Stadium, Hynes Convention Center, Natick Mall and Danvers Doubletree - will close by the end of June.