Especially as the vaccine is emergency authorization use, as per federal law they can't force you to take an experimental treatment. Also employers in general hate dealing with lawsuits, I've seen it in the past where someone who got fired under flimsy reasons was re-instated not even a week later once the company lawyer had a word with that manager. The person got a new job a few months later anyhow but since that incident seems that all managers were more hesitant to fire anyone without really, really good reason.