This study will investigate whether behaviourally informed changes to the invitation process can improve uptake of childhood influenza vaccine by two and three year olds at primary care.

Previous research has shown that small changes to routine invitations/reminders to attend local health services informed by behavioural science can be used to increase a desired behaviour (e.g. uptake of health checks, reducing missed appointments).

This trial will determine whether an invitation letter, informed by behavioural insights and sent through a central system (Child Health Information System, CHIS), can increase uptake of childhood flu vaccine in primary care.

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.

The analysis will investigate the main effect of the intervention on uptake of the flu vaccination for all eligible children included in the trial. The model will include primary care practice and Clinical Commissioning Group effects to account for clustering. Secondary analysis will investigate the impact of such individual factors as age, immunisation history and socioeconomic status and practice-level factors (e.g. direct communication from practices) on uptake of the flu vaccination.

Inclusion Criteria:

• 2 years or 3 year olds at 31st August 2016
• Registered with a primary care practice in the participating area