Updated at 2:01 p.m.

Massachusetts residents ages 65 to 75 and those with more than one health condition that puts them at risk of deadly complications from COVID-19 will become eligible to receive a COVID-19 vaccine Thursday, according to state health officials.

Gov. Charlie Baker announced the next move — officially known as step two of Phase 2 — of his vaccine roll-out plan at a noon Wednesday press conference.

"Over a million people will now be eligible. But if we continue to only get 110,000 first doses of vaccine each week from the feds, it will take a while — at least a month — for everybody to get appointments and get their first vaccination," Baker said.

The Republican governor's vaccination program has been subjected to intense criticism by Democrats and some public health advocates who have said that his efforts don't adequately address the needs of the elderly and the disadvantaged.

Baker defended his vaccine strategy Wednesday, saying that his initial plan targeting specific vulnerable categories instead of large-scale general populations was "very deliberate and very particular, and I would describe [it] as very equitably-framed."

"But the big message we got from the public was 'Vaccinate! Vaccinate!' And there's no question the fastest way to do this is with high volume sites," Baker said.

Health and Human Services Secretary Marylou Sudders said over 50% of the state's 75-and-older population has received the first dose of the vaccine. With news that federal shipments of the vaccines will increase soon, the administration is moving on to the next step of the vaccination plan.

Sudders told reporters Wednesday that though supplies of the vaccine from the federal government are increasing, adding around a million new eligible individuals to the vaccine program means it could take a month or more to book an appointment.

"Unless we see a massive increase in shipments from the feds, it will take us at least a month for people in these new groups to be able to book their first vaccine appointment," Baker said, adding that "the demand is so much greater than the supply that we're getting at this point in time."

The state will use the Centers for Disease Control and Prevention's list of approved health conditions, as well as asthma, to determine eligibility. Conditions include cancer, chronic kidney disease, smoking, Downs Syndrome, various heart conditions, type 2 diabetes and obesity.

Sudders said the state has received word from federal officials that Massachusetts can expect to receive 139,000 first doses of vaccine per week, a 26% increase over the 110,000 doses the state has so far been receiving.

According to Sudders, 70,000 new vaccination appointments will be available tomorrow at the state's mass vaccination sites. The state is also accelerating eligibility for residents and staff of low-income and affordable senior housing.

The state companion program — which allows ineligible individuals accompanying an eligible person to a mass vaccination site to also receive a shot — will not apply to the newly eligible categories and will continue to apply only to those 75 and older.

Step three of Phase 2 is set to include early education and K-12 workers, food and grocery workers, utility workers and some public employees like transit and public health workers. Step four will include any individual with a single threatening medical condition, also known as a comorbidity.

Phase 3 of the plan, still expected to begin as early as this spring if federal vaccine supplies increase, will grant access to the general public.