While several vaccines currently show 90+% effectiveness at preventing infections in the vaccinated person, we still have no data on the other important point: the effect on transmission rates.
Prof Sahin's (one of the developers of the "Pfizer" vaccine) "best guess" (his words) just over a week ago was 50% reduction in transmission rate (i.e. 50% of vaccinated people can still spread it after being vaccinated, presumably the 10% who still get infected + 40% of those who haven't shown measurable signs of infection).
The practical effects of assuming 50% transmission reduction for those vaccinated are:
- The NHS overload criteria is based on potential admission rates from a potentially smaller population: the sum of circa 10% or less of the vaccinated number (at that point in time) and the unvaccinated number (at that point in time). Which will still be a surprisingly large number until you have done lots and lots of vaccination as you are relying on transmission reduction through the recipient not getting infected due to having been vaccinated much more than reducing spreader numbers. (In the early stages assuming no one has lost the immunity they have gained to simplify things)
- Pushes R down to 1.3-1.5 with every one vaccinated and with no other measures in place, hence some other measure will need to remain in place to get / keep R down below 1.
- Means you need to vaccinate most of the adult population not just the high risk groups to bring down effective transmission rates, this means the benefits of vaccination don't accrue that quickly
Hence why you see some caution expressed by expert about the time taken to realise the benefits, "summer" was Prof Sahin's estimate.
The data for effectiveness in transmission reduction will most likely come from Phase 1 of the vaccination programme as you need huge number of vaccinated people to do this in short time scale. Hence no public phase 2 plan yet as the government will want to see what the transmission rate reduction actually is before deciding on the scope and detail of phase 2.