Flu, caused by the influenza virus, is a major health threat every winter. A multidisciplinary scientific team in the UK Health Security Agency (UKHSA), working with our clinical, scientific and public health partners, have brought together big data systems and advanced virological testing to provide new insights into flu vaccine protection. The team generates information for the Joint Committee on Vaccination and Immunisation (JCVI), the United Kingdom’s (UK) independent vaccine advisory committee, that becomes operational clinical policy to give the best protection against deadly winter epidemics.