The biggest reason to predict severe efficacy will be stable (up to the point of antigenic drift where ADE "might" (I'm betting will) come into play) is the drop in infection efficacy itself. The shots are failing in the same way the Salk vaccine did - they inhibit shedding into the bloodstream, but don't encourage durable mucosal IgA antibody levels and thus don't prevent symptomatic infection.

Given this explanation for the Covid vaccine failure, you can either bet that competent humoral antibody ramp-up is *already* being demonstrated by the current outcomes, or wait for further proof in the form of a post 4-months "breakthrough" infection antibody study. I'm inclined to take the bet, and the Lancet authors speaking out against boosters seem to feel the same way.

Again, all this goes out the window once ADE comes into play.