Interim influenza vaccine effectiveness: A good proxy for final estimates in Spain in the seasons 2010-2014

Introduction: The agreement between interim and final influenza vaccine effectiveness (VE) estimates would support the use of interim assessments as a proxy for final VE results to guide health authorities in influenza prevention. We aimed to compare interim/final VE estimates in Spain. Methods: We used a test-negative case-control study (cycEVA) for 2010/11-2013/14 seasons. Sensitivity analyses were carried out by type/subtype of influenza virus and by target groups for vaccination. Results: In general, interim estimates were higher compared to end-season estimates. Interim and final VE differences were higher for the target groups compared to all population. Subtype-specific interim/final VE estimates showed greater concordance (3-13%) than for any virus (7-24%). Conclusion: In Spain, interim influenza VE estimates over 2010-2014 were a good proxy of the final protection of the vaccine. Interim and final estimates showed greater concordance for all population and if performed subtype-specific. (C) 2015 Elsevier Ltd. All rights reserved.