The relative risk reduction (RRR) for vaccinated personnel was modest and lower than reported elsewhere, but closely compares with ILI rates included in a 2018 Cochrane review. The small RRR and large NNV do not support widening the population of UK AF eligible to receive influenza vaccine. Regimental Medical Officers (RMOs) seeking advice on whether to vaccinate other groups of SP should be aware that this approach offers questionable clinical benefit. The ILI surveillance methodology used in this work could be adapted for syndromic surveillance of other infectious diseases.