The reason for lockdowns is to reduce transmission when cases reach the level that hospitalisations and deaths are so high that the NHS is in danger of being overwhelmed. The hope with vaccines is that the link between cases and hospitalisations/deaths has been weakened to the point where the NHS is in no danger even if infections are as high as they were before. In that scenario, there would be no real justification for a further lockdown. We will know more about how the vaccines have affected the linkage in the next few weeks but the current data looks good.