The page you reference is very weird because it's the only vaccine page on the CDC website that gives a range for the effectiveness numbers; all the others just

The page you reference is very weird because it's the only vaccine page on the CDC website that gives a range for the effectiveness numbers; all the others just give a percentage. I'm not sure what the range means, unless it's a 95% confidence interval or something like that; but I have great difficulty in believing that 95% confidence intervals would be that wide after decades of usage of the vaccine. But in any case, it looks like "effectiveness" has been used differently for the mumps vaccine (or the mumps component of the MMR vaccine) for some time. IIRC, there was a mumps outbreak in Ontario in 2011 that raised some doubts about the effectivness of the mumps component of the MMR vaccine; possibly the ranges are from studies done of that outbreak. But again, if the ranges are 95% confidence intervals (and I can't imagine what else they could be), they are atrociously wide for a disease and a vaccine that has been around this long. It is possible, I suppose, that almost all vaccines really are that uncertain in their effects; but if that's true, it should make us much, much less confident about the wisdom of vaccine mandates in general, and COVID vaccine mandates in particular. If we can still be that uncertain about a vaccine that has been around for decades, how much more uncertain should we be about one that was only discovered last year?