The Baker administration plans to distribute 300,000 COVID-19 vaccines over the next three months to Massachusetts residents and workers who face the highest risks for the virus, starting its immunization rollout by focusing on health care workers, long-term care facilities, first responders and congregate care settings.

In one of the most optimistic signs yet that relief from the gruesome pandemic is on the way, state officials charted a clear course Wednesday to make the long-awaited vaccine widely available, pending approval of its emergency use by the Food and Drug Administration.

The three-phase plan is scheduled to roar into action this month with the first 60,000 doses set to arrive by Dec. 15. About 300,000 doses, including both Moderna- and Pfizer-manufactured vaccines, should arrive by the end of the month.

By the spring, millions more doses should land in Massachusetts, and the general public could start accessing immunization free of charge as soon as mid-April.

The first phase will make vaccines available to those critical to keeping the health care system running and others who are most at risk for COVID-19. It will start with health care workers involved in pandemic response.

“Clinical and non-clinical health care workers doing direct care and COVID-facing care are at the highest risk for COVID-19 exposure due to what they do every day,” Gov. Charlie Baker said at a press conference held about nine months after the pandemic ramped up in Massachusetts. “Providing this group with the vaccine first will protect them from exposure and ensure that they can continue to provide health care to others safely.”

Through the rest of the phase, doses will then become available to long-term care residents and staff, first responders, workers and residents at congregate care settings such as shelters and correctional facilities, home-based health care workers and those in the health field who are not involved in COVID care.

In phase two, aimed to begin around mid-February and make more than 1.9 million doses available, adults with two or more comorbidities that create risks for COVID-19 complications would be first in line, followed by workers in fields such as transit, K-12 and early childhood education, sanitation, and grocery stores, then adults 65 years old and older and individuals with just one comorbidity.

The third and final phase, when the vaccine would become available to everyone else in Massachusetts who falls outside those groups, is targeted for the spring. Baker said the “approximate goal” is April.

All but two hospitals in Massachusetts will receive vaccines from the first shipment, according to Health and Human Services Secretary Marylou Sudders, whether directly from the manufacturers, from the state Department of Public Health, or from other hospitals.

Facilities that get doses directly from Pfizer are already equipped with the necessary “ultra-cold freezer storage” to keep vaccines at -70 degrees Celsius, Sudders said.

They will not be the only sites where residents can access the vaccine, though: as the state builds up its infrastructure and receives more doses, many different health care locations will be equipped to conduct vaccinations.

Crucially, vaccines will be offered for free with no out-of-pocket charges or copayments from insurers.

“When available, all health care institutions, including our hospitals, community health centers and private medical offices will have access to the COVID-19 vaccine for their patients,” Sudders said. “Many medical professionals can administer the vaccines: doctors, nurses, dentists, other licensed clinicians, pharmacists, pharmacy interns, pharmacy techs, paramedics and certified medical assistants under supervision.”

Those living or working in long-term care facilities will not need to travel. Under a federal partnership with CVS and Walgreens, representatives from the pharmacies will go to all long-term care locations to vaccinate staff and residents on-site.

The long-term care industry has been devastated by the pandemic, with nearly two-thirds of the more than 11,000 COVID deaths in Massachusetts occurring in facilities.

Members of the advisory group that developed the plan centered equity as a focus of the effort, acknowledging that the virus has wrought disproportionate damage on communities of color and on lower-income areas.

During the distribution’s second phase, a 20 percent vaccine allocation will go to communities with high social vulnerability and more potent COVID-19 impacts.

“This was complex and difficult work,” said Rev. Liz Walker of the Roxbury Presbyterian Church, a member of the advisory group. “Everybody rolled up their sleeves, put in the time and did what was necessary. Communities of color and at-risk populations will be prioritized throughout this process in order to preserve life and prevent serious complications from COVID-related illnesses.”

Tim Foley, executive vice president of the 1199 SEIU union that represents tens of thousands of health care workers, said the plan to prioritize those on the front lines of pandemic response is “the right call.”

“It is imperative that the healthcare and home care workers who have been the heroes of this healthcare crisis be given the protection they need to continue working safely in all care settings,” Foley said in a statement. “Access to COVID-19 testing and vaccination of workers helps to protect patients, and the communities they live in.”

With Wednesday’s announcement of a plan and timeline, hope is more prominently on the horizon. But officials cautioned repeatedly against easing up prematurely.

Both the Moderna and Pfizer vaccines — the two that appear most likely to get imminent federal authorization, according to Baker — require two doses three to four weeks apart.

Dr. Paul Biddinger, director of Massachusetts General Hospital’s Center for Disaster Medicine and the head of the advisory group, said it could take six weeks after vaccination for an individual to achieve immunity.

Even if all of the outstanding variables such as authorizations and supply chains fall into places, Biddinger said it could take six to nine months to “reach a good chunk of the country” with vaccines.

Those who receive a vaccine should continue to follow the now-familiar COVID precautions such as wearing face masks in public, he said.

“What we know about vaccines is that they dramatically lower your risk of needing hospitalization or dying. They protect you,” he said. “What we don’t know is whether they completely prevent you from getting a low-level infection or transmitting the illness.”

“In other words, I could be vaccinated, but I can have a cold, maybe so small I don’t notice, but that means I put others around me at risk,” Biddinger continued. “Until we get to high enough levels of vaccination, we need to be following the same guidance all together. I can’t take my mask off because I’ve been vaccinated.”

Biddinger also stressed that those who have had COVID already — a group in Massachusetts that includes more than 240,000 people — should not shy away from a vaccine.

Under current U.S. Centers for Disease Control guidance, anyone who has had a confirmed COVID case should wait three months before receiving a vaccine, he said, but those with infections older than that can still gain from the vaccine’s protections.

American Civil Liberties Union of Massachusetts Executive Director Carol Rose praised the Baker administration’s plan for emphasizing equity, particularly because it ensures inmates in correctional facilities — where several major outbreaks have occurred — will be included in the first phase.

“Making vaccines available to incarcerated people is an important step toward containing COVID-19 inside and outside of these facilities and providing constitutionally mandated medical care, but Governor Baker and other leaders should also act now to reduce incarceration levels and increase testing so that more people are alive to receive the vaccine when it becomes available,” Rose said in a statement.