I mean, arguing against human vaccines, a universally good thing, is a hard needle to thread. There's not really any statement to be made against it unless the employer was requiring vaccines and not offering employees reasonable time and opportunity to attain them. There also isn't any reason to expect that a state entity would tell all librarians to go get the smallpox vaccine, one because smallpox now isn't like Covid-19 now, and two because most Boston librarians are, arguably, smart enough to already have been vaccinated.

The one (and only) actual argument to be made here is about if employers have the right to force employees to get medical procedures and under what circumstances. Right now it's fine, but if you actually wanted to make this argument you'd likely say that, it may be in the future there's a medical procedure where all USPS employees need to have mail canons installed on their arms to more efficiently deliver mail.

It's a stupid argument, and also the only one that half-makes sense.