OK, so here's some real-world data reported today:
Israel is reporting that boosters reduce infection significantly in the over-age-60 demographic: 86% and 92% for severe covid.
Not sure if that's baseline over non-vaccinated, or over the original vaccine regimen.
It's not clear (and they admit it) whether the drop in efficacy was due to antibody decline or delta. Delta is a separate issue.
Article behind the WSJ paywall.