I am not sure what all might have been in the bill or pending in other bills they did not want to piss the Republicans off about, that the issue wasn't forced. You know the republican wouldn't have voted for it, if it killed them. The Democrats, did not force it, they just went ahead and all got vaccinated, preferring to live, I suppose, rather than roll the dice.
Congress exempts themselves from laws that cost them money, or that endanger them.