Just thinking out loud here, but does a vaccine have to be long-acting?
I mean, in an ideal world, it would be. But if the main goal is to get R0 down under 1 for long enough for this to fizzle out, then even if it gave immunity for a few months, it would be a massive help.
If (big if) you could vaccinate a very large proportion (say 80%) of the population over a short period of time, then have a sort of lockdown-lite for a couple of months, plus heavy-duty testing and contact-tracing, plus masks, etc, is it not likely that we could get infection rates down super-low - to the point they could be managed on a long-term basis with testing and contact-tracing?
For highly vulnerable groups (e.g. frontline NHS staff and those in care homes) could you re-vaccinate every few months? I'm guessing there are risks associated with that, but possibly lower risks than catching covid.