Effects of previous episodes of influenza and vaccination in preventing laboratory-confirmed influenza in Navarre, Spain, 2013/14 season

We estimated whether previous episodes of influenza and trivalent influenza vaccination prevented laboratory- confirmed influenza in Navarre, Spain, in season 2013/14. Patients with medically-attended influenzalike illness (MA-ILI) in hospitals (n = 645) and primary healthcare (n = 525) were included. We compared 589 influenza cases and 581 negative controls. MA-ILI related to a specific virus subtype in the previous five seasons was defined as a laboratory-confirmed influenza infection with the same virus subtype or MA-ILI during weeks when more than 25% of swabs were positive for this subtype. Persons with previous MA-ILI had 30% (95% confidence interval (CI): −7 to 54) lower risk of MA-ILI, and those with previous MA-ILI related to A(H₁N₁)pdm09 or A(H₃N₂) virus, had a, respectively, 63% (95% CI: 16–84) and 65% (95% CI: 13–86) lower risk of new laboratory-confirmed influenza by the same subtype. Overall adjusted vaccine effectiveness in preventing laboratory-confirmed influenza was 31% (95% CI: 5–50): 45% (95% CI: 12–65) for A(H₁N₁) pdm09 and 20% (95% CI: −16 to 44) for A(H₃N₂). While a previous influenza episode induced high protection only against the same virus subtype, influenza vaccination provided low to moderate protection against all circulating subtypes. Influenza vaccine remains the main preventive option for high-risk populations.