Massachusetts is sorting through the challenges of mass vaccination as it prepares to roll out a program to millions of people in Massachusetts’ general public.

The state is still in Phase 1 of its 3-phase rollout of the COVID-19 vaccine, offering shots to health care professionals, residents and staff in congregate care settings, and first responders. Phase 2 is expected to begin next month and will include seniors and people with two risk factors, called comorbidities. The general public is expected to start getting vaccinated in Phase 3 in April. And as the state prepares, challenges abound, including determining appropriate locations for vaccination clinics and finding people to administer the vaccine.

The first challenge is to make sure there’s enough vaccine — knowing how much is coming and when. Gov. Charlie Baker said this has been a complication of the phase 1 rollout.

“Frankly, at this point, we're typically hearing when the next dose is coming a couple of days before it lands,” Baker said at a recent press briefing.

Not knowing when the vaccine will arrive makes it hard for vaccination sites to book appointments ahead.

Dr. Paul Biddinger, chair of the state’s vaccine advisory group, told GBH News he’s hoping for a more predictable supply in the coming weeks.

“But right now, it's just one of so many unknowns that has made this rollout, this planning so very hard,” he said.

Biddinger said another complication of the effort is finding space for vaccinating a large number of people.

“There's a required 15-minute observation period after people get vaccinated, which creates a huge space need," he said. "So that after you've been vaccinated, we have to create a large number of chairs, six feet apart for people to be observed before they can move on."

The state is looking for large venues with plenty of space — Baker announced Tuesday that Fenway Park will become Massachusetts’ second mass vaccination site for the first phase after Gillette Stadium, which is already home to a site. Biddinger said they’re also considering big drive-through sites, and the state has also begun distributing the vaccine to independent physicians' practices. CVS and Walgreens are also receiving doses beginning this week. A CVS spokesperson told GBH News that any in-store vaccinations will require an appointment.

Boston’s chief of health and human services, Marty Martinez, said finding appropriate vaccination venues is just one of many challenges the city is still working out.

“How do you ensure you’re following the prioritization that’s been laid out so those most vulnerable get vaccinated first?” he asked. “Making sure that in diverse communities, these clinics are accessible, in neighborhoods where people may have hesitancy to get vaccinated? So the city’s working through all of that.”

Community health centers are also expected to play a major role in vaccine distribution.

“If there's a trusted voice in communities that can get past this vaccination hesitancy, it’s community health centers,” said Michael Curry, president and CEO of the Massachusetts League of Community Health Centers.

Lines are already wrapping around the block at many community health centers for COVID-19 testing. Curry said they’re now trying to find the resources to support the vaccination effort, and he’s hoping Congress will step in to help provide it.

“Imagine in a system that's already overwhelmed, with a workforce that's already overwhelmed, with workforce shortages that existed well before this pandemic, that we're adding on top of that a massive vaccination effort,” Curry said.

Another challenge is finding enough people to administer the vaccine.

“I don't know that there's a shortage of people that are qualified to do it,” said Dan Hoffenberg, vice president of Cataldo Ambulance Service, which has been recruiting people for the job. “I think there is a shortage of people being asked to do it.”

Hoffenberg said they’ve gotten about 1,500 applications and will start in-person training this week.

There are also hundreds of medical volunteers signing up to help their local communities host vaccination sites.

Health departments in many small towns don't have the capacity to host their own vaccination clinics, but Dr. Marcia Testa, president of the Massachusetts Association of Health Boards, said many plan to partner with neighboring communities.

“I think people don't realize how much training health departments have been going through every month, having meetings with their public health preparedness trainers and managers,” Testa said. “And so now it's actually putting into practice what you've been drilling on for almost 15 years.”

Vaccinating an entire state is a huge endeavor. But Testa said she’s optimistic it’s going to wind up being a lot more coordinated than most might expect.