A study led by the University of Birmingham in collaboration with Public Health England, found that extending the interval to 12 weeks increased the peak spike-specific antibody response to the virus 3.5-fold compared with those who had the second vaccine at three weeks.
Although the peak cellular immune responses were lower after the delayed second vaccine, responses were comparable between the groups when measured at a similar time point following the first dose, researchers found.
There has been some debate about the UK government's strategy of giving second jabs at 12 weeks rather than three, a strategy which was originally adopted to protect as many of the most vulnerable as possible as quickly as possible. It was initially controversial though as evidence mounted that a single dose has substantial benefits within three to four weeks more and more experts around the world have come out in favour of the strategy.
I understand this is the first study to compare the immune response and it does appear to provide strong evidence that the policy of giving the second dose at 12 weeks is the best one.
You can read a summary of a Telegraph article about the study here.