Rather than say “Hmm... Maybe the vaccine isn’t protecting like we think it is”, they said “The vaccine only works if everyone is vaccinated, otherwise an unvaccinated carrier can make everyone sick! BAN THE NON VAXXERS!”
It takes a bit of mathematics to understand what is going on here. Few, if any, vaccines are 100% effective in every recipient. Thus, if a vaccine is 97% effective, 3 people out of every 100 exposed will still get sick. On the other hand, the majority of non-vaccinated people who are exposed will get the disease. So you can have an outbreak situation where 8/10 unvaccinated get the disease and 30/1000 vaccinated people get the disease. In absolute numbers, more vaccinated people got sick, but the rate of illness was only 3% among the vaccinated as compared to 80% among the unvaccinated.
The problem was that not one case of mumps was found to be in an unvaxed person. Not one.
The first pass was that it was in all unvaxed kids (we are in the Chiropratic center of the world here in Eastern Iowa, so that is a reasonable first guess). However, they didn’t find a single case. They traced it to a 4 year old girl at a local preschool, but were not able to trace it back from there. Her family hadn’t traveled out of the area in that time, and had no unusual exposure.
So we had 8 kids with mumps, all in a population of under 50, all in a cluster around the preschool and elementary school in my town. All had current vaccinations. That is well beyond the expected failure rate of vaccines. It is (if I remember right) about three times more than the expected failure rate. And there were no cases of unvaxed people with the mumps in the area. Possible that the exposure was in a transit point (gas station, Walmart, etc), but again they didn’t find the source of the first little girl getting it. Even among the illegal alien population.
I grew up on a farm. We knew, studied, and lived vaccinations. Dad kept very careful records of sick animals, and if he saw a cluster like this it would be a cause for revisiting the vaccine. Or a worry that some other bug just got into the population.
That is a big red flag that some thing is wrong. Now one theory, that makes a bit of sense, is that there was a bad batch of vaccine. This would explain some of the cluster, but there were kids older than the index cases that got it, and not one of the older people (parents, grandparents, etc.)