Introduction
FLU, FLU, INFLUENZA
In 2012, the UK Joint Committee on Vaccination and Immunisation recommended a national childhood influenza immunisation programme be rolled out to 2- to 16-year olds, using a live attenuated influenza vaccine [In 2015/16 nationally, uptake in school years 1 and 2 was 53.6% [National template letters are published on the Annual Flu Programme website,Much of the literature on vaccine uptake focusses on vaccine hesitancy and on knowledge, attitudes and beliefs about vaccines (e.g. [Applying behavioural science to invitation letters can increase the uptake of health interventions. For example, a 4% increase in the uptake of the NHS Health Checks was achieved by simplifying the invitation letter and adding a tear-off slip where individuals could note the time and date of their first appointment, as a memory aid [Reminders can also increase uptake of interventions, with text message reminders having been successful at increasing uptake of NHS Health Checks [In 2016/17 Public Health England Behavioural Insights (PHEBI) worked with NHS England and Public Health England (PHE) immunisation team to redesign the template invitation letters for general practice and primary schools. In this paper we report on two RCTs, the first clustered at the GP level, and the second at school level, to test whether behaviourally-informed letters can increase uptake of the childhood influenza (flu) vaccination. In both cases, clustering was a pragmatic choice to deliver the intervention and reduce the potential for cross-treatment contamination.