Effectiveness of subunit influenza vaccination in the 2014-2015 season and residual effect of split vaccination in previous seasons

Background: In Navarra, Spain, subunit vaccine was first used in the 2014-2015 season, whereas trivalent split-virion influenza vaccines had been used in previous seasons. We estimate the effectiveness of the subunit vaccine in the current season and split vaccine in the two previous seasons against laboratory-confirmed influenza in the 2014-2015 season. Methods: Patients with influenza-like illness hospitalized or attended by sentinel general practitioners were swabbed for influenza testing. The previous and current vaccine status of laboratory-confirmed cases was compared to test-negative controls. Results: Among 1213 patients tested, 619 (51%) were confirmed for influenza virus: 52% influenza A(H3N2), 46% influenza B, and 2% A(H1N1)pdm09. The overall effectiveness for subunit vaccination in the current season was 19% (95% confidence interval [CI]: -13 to 42), 2% (95%CI: -47 to 35) against influenza A(H3N2) and 32% (95%CI: -4 to 56) against influenza B. The effectiveness against any influenza was 67% (95%CI: 17-87) for 2012-2013 and 2013-2014 vaccination only, 42% (95%CI: -31 to 74) for 2014-2015 vaccination only, and 38% (95%CI: 8-58) for vaccination in the 2012-2013, 2013-2014 and 2014-2015 seasons. The same estimates against influenza A(H3N2) were 47% (95%CI: -60 to 82), -54% (95%CI: -274 to 37) and 28% (95%CI: -17 to 56), and against influenza B were 82% (95%CI: 19-96), 93% (95%CI: 45-99) and 43% (95%CI: 5-66), respectively. Conclusion: These results suggest a considerable residual protection of split vaccination in previous seasons, low overall effectiveness of current season subunit vaccination, and possible interference between current subunit and previous split vaccines.