Effect of previous and current vaccination against influenza A(H1N1) pdm09, A(H3N2), and B during the post-pandemic period 2010-2016 in Spain

Background Recent studies suggest that the protective effect of the current influenza vaccine could be influenced by vaccination in previous seasons. We estimated the combined effect of the previous and current influenza vaccines from the 2010-2011 season to the 2015-2016 season in Spain. Methods We performed a test-negative case-control study in patients >= 9 years old. We estimated the influenza vaccine effectiveness (IVE) against influenza A(H1N1) pdm09, A(H3N2), and B virus. Results We included 1206 influenza A(H1N1) pdm09 cases, 1358 A(H3N2) cases and 1079 B cases. IVE against A(H1N1) pdm09 virus in the pooled-season analysis was 53% (95% Confidence Interval (CI): 21% to 72%) for those vaccinated only in the current season and 50% (95% CI: 23% to 68%) for those vaccinated in the both current and previous seasons. Against the influenza A(H3N2) virus, IVE was 17% (95% CI: -43% to 52%) for those vaccinated only in the current season and 3% (95% CI: -33% to 28%) for those vaccinated in both seasons. Regarding influenza B, we obtained similar IVEs for those vaccinated only in the current and those vaccinated in both seasons: 57% (95% CI: 12% to 79%) and 56% (95% CI: 36% to 70%), respectively. Conclusion Our results suggested no interference between the previous and current influenza vaccines against A(H1N1) pdm09 and B viruses, but a possible negative interference against A (H3N2) virus.