Whilst you're right that the effectiveness of the vaccine should be monitored, the vaccine has been approved for use in the UK.
On the strength of that, at this particular juncture, it would seem perfectly reasonable to make some kind of plan / prediction for the months ahead. In every industry plans, timescales, deadlines, programmes etc are worked to - granted there's sometimes hiccups along the way. But I don't see why this should be any different really.