The trial will take place within the existing national childhood immunisation programme in the participating area in England. Randomisation will be clustered at the primary care practice level. Outcome data will be anonymised, routinely collected, individual-level influenza vaccine uptake data extracted from CHIS. For CHIS data validation purposes, practice-level vaccine uptake data reported through an alternative, routine system will be collected. Data will be collected on additional invitations/communications that primary care practices (in both the intervention and control arms) send to their patients. Cost data for the centralised letter will also be obtained.