Or is it all too difficult to predict with the vaccinations?
Yes, I think it is, the real problem is vaccination breakthrough, the numbers of cases in the under 30's and not in a vulnerable group could be really high and hospitalisations would remain very low.
However a small amount of vaccine breakthrough to a serious case would rapidly become significant.
At the moment, there appears to be very little vaccine breakthrough towards serious cases (there's just been a few such deaths, and hundreds of this group die every day so some will be with covid, or covid ending their struggle the same as any other infection would have)
So I don't think we can say a number that would be an absolute concern.