Randomisation
FLU
A stratified randomisation strategy was used. In total 14 strata were created (all of the possible combinations of the seven CCGs and whether or not they were SystmOne users: SystmOne is a software system used in some practices; the databases of practices with SystmOne were automatically updated with a copy of the letter sent on the patient’s record and any flu immunisations recorded on the GP practice system were automatically transferred to CHIS, whereas other practices may have done this manually). Randomisation was by general practice. General practices within each strata were randomly assigned to the intervention group (Group 1) or control group (Group 2). A code was developed in Stata by researchers at PHE to randomise practices. General practices in the intervention group were informed that CHIS was sending an invitation letter to the parents of two- and three-year olds as part of a trial to try to increase immunisation uptake. Parents were not informed that the letters were being sent as part of a trial. General practices in the control group were not informed about the trial.