As Ive said before, the mumps vax is around 70-75% effective. There’s little breakthrough infection because there’s not much virus circulating. Covid vaccines are effective, but there’s still a ton of virus out there. Mumps doesn’t circulate much because most are vaccinated. Measles circulate too. Once vax rates go down in spots, you get an outbreak, even in some vaxed people. Contrary to the opinion of some here, I think masks are mostly useless except in certain narrow settings. But the more people are vaxed, the less it’ll circulate. Not hard to understand.