title: Impact of influenza B lineage-level mismatch between trivalent seasonal influenza vaccines and circulating viruses, 1999-2012.
abstract: BACKGROUND
  Influenza B virus strains in trivalent influenza vaccines are frequently mismatched to the circulating B strains, but the population-level impact of such mismatches is unknown. We assessed the impact of vaccine mismatch on the epidemiology of influenza B during 12 recent seasonal outbreaks of influenza in Finland.
  
  
  METHODS
  We analyzed all available nationwide data on virologically confirmed influenza infections in all age groups in Finland between 1 July 1999 and 30 June 2012, with the exclusion of the pandemic season of 2009-2010. We derived data on influenza infections and the circulation of different lineages of B viruses during each season from the Infectious Diseases Register and the National Influenza Center, National Institute for Health and Welfare, Finland.
  
  
  RESULTS
  A total of 34 788 cases of influenza were recorded. Influenza A accounted for 74.0% and influenza B for 26.0% of all typed viruses. Throughout the 12 seasons, we estimated that 41.7% (3750 of 8993) of all influenza B infections were caused by viruses representing the other genetic lineage than the one in the vaccine. Altogether, opposite-lineage influenza B viruses accounted for 10.8% of all influenza infections in the population, the proportion being highest (16.8%) in children aged 10-14 years and lowest (2.6%) in persons aged ≥70 years.
  
  
  CONCLUSIONS
  The population-level impact of lineage-level mismatch between the vaccine and circulating strains of influenza B viruses is substantial, especially among children and adolescents. The results provide strong support for the inclusion of both influenza B lineages in seasonal influenza vaccines.
authors.affiliation: Department of Pediatrics, University of Turku and Turku University Hospital.
authors.forename: Terho
authors.initials: T
authors.lastname: Heikkinen
authors.affiliation: National Influenza Center, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Niina
authors.initials: N
authors.lastname: Ikonen
authors.affiliation: National Influenza Center, National Institute for Health and Welfare, Helsinki, Finland.
authors.forename: Thedi
authors.initials: T
authors.lastname: Ziegler
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D018563:Disease Notification; D005387:Finland / Q000453:epidemiology; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D009980:Influenza A virus / Q000276:immunology*; D009981:Influenza B virus / Q000276:immunology*; D007252:Influenza Vaccines / Q000008:administration & dosage / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology / Q000517:prevention & control* / Q000821:virology*; D032541:Mass Vaccination; D008875:Middle Aged; D012189:Retrospective Studies; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines
keywords: influenza B virus; quadrivalent influenza vaccines; trivalent influenza vaccines; vaccine mismatch