Reducing vulnerable people's risk by 70% is great...but not enough to change pretty much anything for them personally, like being able to see family if in a care home, go out more if they are very elderly. They are still at some considerable risk.
Also, drop out at 12 weeks will be higher than at 4 weeks, people have the momentum, but communication/logistics 12 weeks later, when they are already vaccinating large numbers of others will just mean many people don't get the dose now.
Pfeizer couldn't run a hugely long trial which is probably why the dosing regime was 4 weeks apart, given the pressure to produce an effective vaccine, but it does now mean that we are not following the manufacturer's instructions which I think is very very foolish, I think the first million people should have been vaccinated with the four week schedule, you could then have a baseline (although they are not in a trial) to consider departing from this. It's not even clear the vaccines work in the real world how they did in the lab, as the groups tested in the trials are pre-selected, and screened, so no bad allergies, no really old people- we have no idea how the jab will or won't work in these populations, so that should have been the first thing to establish, not whether the first dose protection holds for less than 12 weeks...
It is a slow and steady wins the race here. I don't agree with them departing from the schedule, because I think it's debatable whether vaccinating more at 70% efficacy will really produce any step change in behaviour, ability to socialise, anything- the first batch should have been given exactly as specified in the wider real world population and then, three months later once we had some initial idea of how that went, a mass vaccination using only one dose could have been in place.
This is a long game, it is not at all true that this will be over by Easter for all kinds of complex logistical and psychological reasons.