You'll probably find the strains are different and that is why there is a difference, The entire vaccine is already dead so wont die before it can work, but I get what you mean. It's why they tailor flu vaccines for different continents, because the flu strains causing the outbreaks are different.
Yeah. Its like milk going off in a fridge that has a bung seal iykwim. There is something. There was studies in QLD that said the most common strain was covered by out DTaP, but they are a few years old, before this new major outbreak. It really is hard to say for sure but they still insist on blaming anti-vaxxers.

Do you think it could also be that they are doing so many vaccines at once? These days it's a 5 in 1, not a 3 in 1. Too many at once?

Also a lot of people seem to suffer from Candida these days, systemic yeast overgrowth can effect the cells from properly enveloping bacteria and virus'.
In NSW we use Hexa - which is 6 in one needle, plus one other. This is why I delay vax. 7 virii at once is an incredible assault on a 2mo baby, IMO.

That is an interesting point. I wonder what the rates of oral thrush are like in babies?