Why aren't flu vaccines leaky and having the same potential issues as we're seeing with covid?

Also, I'd not caution when looking at UK data - the number of tests reported include LFT taken at home as well as PCR. It's only this year that we've been asked by govt to test (and report) twice a week when we don't have symptoms with a LFT, so comparing year-on-year, unless you're only looking at data form PCR tests only won't be comparable.