What evidence is there about efficacy of 2nd dose after 12 weeks?

[notevenat20]

We now have good evidence that for the Oxford vaccine at least you are well protected for the first 12 weeks by one dose. This is great. Scientifically speaking, why do we believe the second dose at 12 weeks will be as effective as a second dose at 3 weeks? Is there any evidence either way?

It says on BBC that the second dose at 12 weeks increases the effectiveness from 76% to 82% which is higher then with a shorter gap, though it didn't say how effective it is with 4 week gap, just that 82% was an improvement.

@ForeverBubblegum

Thanks. Maybe that is somewhere in the paper they are talking about.