Early estimates of influenza vaccine effectiveness in Navarre, Spain: 2012/13 mid-season analysis.

We present estimates of influenza vaccine effectiveness (VE) in Navarre, Spain, in the early 2012/13 season, which was dominated by influenza B. In a population-based cohort using electronic records from physicians, the adjusted VE in preventing influenzalike illness was 32% (95% confidence interval (CI): 15 to 46). In a nested test-negative case-control analysis the adjusted VE in preventing laboratory-confirmed influenza was 86% (95% CI: 45 to 96). These results suggest a high protective effect of the vaccine.