Monday is the day a lot of people have been waiting for in Massachusetts.

The state will open up eligibility for the COVID-19 vaccine to anyone 16 or over, adding another 1.7 million people to the list of those who can sign up.

Monday’s registration is the latest test for the state, which has faced challenges with its vaccine supply, distribution capacity and technological readiness. But state officials say they are ready.

For now, the state doesn’t have the Johnson & Johnson (J&J) vaccine in its arsenal, following a national pause of that company’s vaccinations earlier this week.

Public health officials in Massachusetts had expected a weekly influx of about 100,000 doses of the single-shot vaccine. But even before the pause, the J&J supply had been inconsistent, in part because of a manufacturing problem that caused millions of doses to get thrown out.

Gov. Charlie Baker said Wednesday the state has the capacity to administer two or three times more vaccines than it has the supply for.

“And in my mind, I always thought of the J&J as a piece that would be a significant part of sort of amping it to the next level,” Baker said, adding that Massachusetts will begin using the vaccine again if federal officials say it’s safe.

Until then the state will continue to rely on the Moderna and Pfizer vaccines.

“We'll use whatever vaccine is available,” Baker said. “We've demonstrated over the course of the past several months that we can work with whichever product is available to us, and we'll continue to do that.”

But the J&J pause will slow things down.

“Because of limited supply, unfortunately, not everyone can get their first dose on Monday or in the first week,” said Dr. Paul Biddinger, medical director for Emergency Preparedness at Mass General Brigham, and the chair of the state's COVID-19 Vaccine Advisory Group.

Biddinger expects that the shortage of vaccination appointments will only last a few weeks.

“I expect that will change dramatically,” he said. "We've made extraordinary progress vaccinating so many people across the Commonwealth. The same will absolutely be true for everyone who's eligible on Monday. And so people need to be patient, as hard as that is.”

Biddinger acknowledged that's been tough for some in Massachusetts as they’ve watched eligibility open up in other states.

That's partly because other states don't have the same level of demand as Massachusetts, where there's a higher interest in vaccination, Biddinger said. The state's focus on seniors and people like healthcare workers and first responders has paid off with relatively high levels of vaccination, he said.

The expanded eligibility also will be a test for the state's registration website, whichcrashed when it first launched in February, overwhelmed by demand.

“I'm not worried about the website,” Baker said last week, before pausing and rethinking that statement. “That's not fair, I'm always worried about the website. But we've been assured that it won't be a problem and they've run some pretty heavy stress tests on it.”

Tiffany Tate, executive director of the Maryland Partnership for Prevention, which was contracted to run the Massachusetts site, said the coalition has taken steps to improve the site’s function and experience, like adding a waiting room.

“We are excited about the expanded eligibility and have worked with the Department of Health to prepare for increased traffic to the site,” Tate said in an email to GBH News.

The past several weeks also has seen significant expansion in the number of locations available for people to get a vaccine.

Mass vaccination sites continue to operate at places such as the Hynes Convention Center, Gillette Stadium and the Reggie Lewis Center in Roxbury. Patients of community health centers can get vaccinated there. There also are regional collaboratives, where local health departments have banded together to provide vaccines.

Many people also can now get vaccines through their own health care providers. And commercial pharmacies are getting vaccines directly from the federal government, as well as from the state in some places.

The state’s vaccine registration website lays out the various options, once users enter their zip codes.

Several independent online resources have sprung up to help people find available vaccines, including the Twitter account @vaccinetime, which regularly tweets out where appointments are available. Other sites like and are aggregating data on vaccine availability to help people make appointments.

With more people eligible, Biddinger said he doesn't expect it will be hard to vaccinate 50 percent of people, but getting those percentages closer to the 70 percent or so needed for herd immunity will require outreach and education.

And there’s one more large group that is still not eligible for the vaccine, and that’s kids under 16. But that could change soon. Pfizer recently announced positive trial results for kids ages 12-15, and has said it plans to ask the FDA to expand eligibility to that age group.

That expansion of eligibility could happen “within the coming weeks,” Biddinger said, and Moderna may not be far behind.