The move comes the morning after Pres. Biden said he was telling states to add school workers to the top of their priority lists, something Gov. Baker had steadfastly refused to do.
CVS can presumably do this because it gets its vaccines directly from the feds, rather than the state. The move doesn't necessarily mean they have any more vaccine, though. This site checks CVS (and Walgreens and Rite Aid) and can notify you when slots open up.
At a press conference, Baker said the state will follow suit on March 11, but that the state needs more vaccine.