BOSTON On the first day residents 75 and older could sign up for the COVID-19 vaccine, Gov. Charlie Baker said he knows people are frustrated about the time it takes to get those appointments, but the commonwealth continues to be constrained by the supply of vaccines on hand. I think the biggest challenge we re going to face on this rollout, and we ve said this several times, is if demand does outstrip supply, which is where we re going to be for some period of time until the federal government can get to the point where their distribution to us reaches some level that s consistent with the number of people who are eligible to get vaccinated, Baker said in his daily press availability on Beacon Hill.