Intervention 1: Behaviourally-informed letter
FLU
A behaviourally-informed letter was sent via post by providers of childhood flu school vaccination to parents at those schools randomised to Intervention 1, with minimal local adjustments. The behavioural techniques used were the same as the ones used in the general practice trial except there was no personalisation (this was not feasible, since exactly the same letter is sent to all parents) or a tear-off appointment time slip (because the required action was returning a consent form, not booking an appointment). The schools letter also included a positive social norm statement (Behavioural insights informed changes to the flu invitation letter and rationaleParents at schools randomised to a control letter group were given usual practice, a standard letter invitation letter sent to all parents from the provider. They were sent the letter used by the immunisation provider during the previous year (with necessary updates). These letters varied from area to area.