Scientist Andrew Pollard who co-developed the AstraZeneca vaccination together with Sarah Gilbert at Oxford University has said that the ideal gap between the doses are Three months to provide very good protection.
This gets even better with a longer interval from three to four months. A longer time gap gets a stronger immune response after the second dose.
The reason for the UK’s change is the situation posed by the B.1.617 variant and the attempt to improve immunity and curb transmission. But to get the maximum immune response, it’s worth waiting longer.
There is a trade-off between getting a high response earlier or waiting longer and getting an even better immune response.