Influenza Vaccine Effectiveness: Analysis of the Impact of Repeated Vaccinations in Military Health System Beneficiaries.

Background: Influenza has long burdened the Military Health System (MHS). This study assesses the impact of repeated annual vaccination on influenza vaccine effectiveness (VE). Methods: This retrospective, case control study using the test-negative design utilized data extracted from the MHS Data Repository (MDR). Cases had a positive influenza test and controls sought care for an influenza-like illness within 2 weeks of a case, had no positive influenza tests, and were matched by sex, race, age, and location. Vaccine effectiveness was assessed using conditional logistic regression separately for those who received inactivated and live attenuated influenza vaccines (LAIV). Results: A total of 6860 cases and controls were identified in the MDR, among whom 53% were vaccinated in all 3 seasons. Among those who received inactivated influenza vaccine during the current season, VE ranged from 26% to 37% (2012/13 [A(H3N2)]: VE 26%, 95% confidence interval [CI] = 1%-45%; 2013/14 [A(H1N1)pdm09]: VE 37%, 95% CI = 18%-52%; 2014/15 [A(H3N2)]: VE 31%, 95% CI = 17%-42%). The VE ranged from 25% to 49% for those only vaccinated this season (2012/13 [A(H3N2)]: VE 38%, 95% CI = -3% to 63%; 2013/14 [A(H1N1)pdm09]: VE 49%, 95% CI = 11%-71%; 2014/15 [A(H3N2)]: VE 25%, 95% CI = -7% to 48%). The VE was more variable in those who received LAIV in the current season. No statistically significant differences in VE were observed between those frequently vaccinated and those vaccinated only during the current season. Conclusions: These results underscore the value of annual influenza vaccinations for preventing infection while highlighting the need for continued improvements in influenza vaccine effectiveness.