Effect of repeated vaccination with the same vaccine component against 2009 pandemic influenza A(H1N1) virus

Abstract

Background: The 2009 pandemic influenza A(H1N1) (A[H1N1]pdm09) vaccine component has remained unchanged from 2009. We estimate the effectiveness of current and prior inactivated influenza A(H1N1)pdm09 vaccination from influenza seasons 2010-2011 to 2015-2016. Methods: Patients attended with influenza-like illness were tested for influenza. Four periods with continued A(H1N1)pdm09 circulation were included in a test-negative design. Results: We enrolled 1278 cases and 2343 controls. As compared to individuals never vaccinated against influenza A(H1N1) pdm09, the highest effectiveness (66%; 95% confidence interval, 49%-78%) was observed in those vaccinated in the current season who had received 1-2 prior doses. The effectiveness was not statistically lower in individuals vaccinated in the current season only (52%) or in those without current vaccination and >2 prior doses (47%). However, the protection was lower in individuals vaccinated in the current season after >2 prior doses (38%; P =.009) or those currently unvaccinated with 1-2 prior doses (10%; P 2 prior doses. Conclusion: Current vaccination or several prior doses were needed for high protection. Despite the decreasing effect of repeated vaccination, current-season vaccination was not inferior to no current-season vaccination.