Introduction This paper describes the first ever analysis of health data to report influenza vaccine uptake and the effectiveness of the vaccine in preventing general practice presentations for influenza-like illness (ILI) in the UK Armed Forces (UK AF). This was undertaken during the 2017–2018 influenza season.

Methods Clinical Read codes for ILI and influenza vaccinations were used to generate reports for the period from September 2017 to April 2018. Using a methodology adapted from Public Health England's (PHE) in hours syndromic surveillance, the ILI rate for the UK AF was calculated. Subsequent analysis explored vaccination uptake in target groups and compared the relative risk (RR) of ILI in vaccinated versus unvaccinated Service Personnel (SP).

Results 4234 SPs had a record of ILI between September 2017 and April 2018, with a peak rate of 216 cases per 100 000 PAR. The absolute risk reduction for reporting ILI in vaccinated versus unvaccinated SP was 0.4% (p=0.0031), and the RR was statistically significant at 15% (95% CI 5% to 23 %) lower than in the non-vaccinated PAR. The number needed to vaccinate (NNV) to prevent one presentation of ILI was 241 (95% CI 145 to 714). The 8153 vaccinations recorded for the untrained strength equate to approximately 38% of overall training throughput and 65% of all Army SP recorded as being in phase 1 training at some point during this period.

Conclusions 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.