The A(H1N1)pdm09 influenza vaccine component has remained unchanged from 2009. We estimate the effectiveness of current and prior inactivated influenza A(H1N1)pdm09 vaccination from season 2010–2011 to 2015–2016.
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.
We enrolled 1278 cases and 2343 controls. As compared to individuals never vaccinated against influenza A(H1N1)pdm09, the highest effectiveness was observed in those vaccinated in the current season who had received 1–2 prior doses (66%; 95% confidence interval 49%–78%). The effectiveness was not statistically lower in those vaccinated in the current season only (52%), or in individuals 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 currently unvaccinated with 1–2 prior doses (10%; P < .001). Current season vaccination improved the effect in individuals with 1–2 prior doses, and did not modify significantly the risk of influenza in individuals with >2 prior doses.
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 vaccination.