Davis, T. Quick question: when do we immunise premature babies, Don't Forget the Bubbles, 2015. Available at:
When babies are born prematurely, at what age do they get their immunisations?
Babies should receive their immunisations based on their actual age (not corrected age). So if a baby was born at 26/40 and is four months old today (i.e. it is four months since they were born), they should receive their four month immunisations today.
As a rule: weight and development are based on corrected age; birthdays and immunisations are based on actual age.
See the immunisation schedule here.