Circulation of Influenza Type B Lineages in Greece During 2005-2015 and Estimation of Their Impact.

The past decades influenza B lineages Victoria and Yamagata cocirculated. Our aim was to estimate the distribution of the two lineages circulating in Greece and any possible mismatching with vaccine influenza B strains. We studied 490 laboratory-confirmed influenza B nonsevere acute respiratory infection (non-SARI) cases diagnosed in the two National Influenza Reference Laboratories by reverse transcriptase polymerase chain reaction from July 1, 2005 to June 30, 2015 and 100 influenza B SARI cases diagnosed from July 1, 2011 to June 30, 2015. Median matching between the circulating influenza B lineages and the vaccine influenza B strains was 19.2% (range: 0-100%) for non-SARI cases during 2005-2015 and 67.6% (range: 41.2-94.1%) for SARI cases during 2011-2015. In two influenza seasons (2005-2006 and 2006-2007), complete lineage mismatch between influenza B non-SARI cases and influenza B vaccine strains was found. We estimated that 5, 12, or 16 laboratory-confirmed SARI cases could have been prevented by quadrivalent influenza inactivated vaccine (QIV) during the 2011-2012 season and 1, 2, or 3 SARI cases during the 2014-2015 season, with a vaccination coverage rate of 70% and a vaccine effectiveness of 20%, 50%, or 70%, respectively. Significant cocirculation of Victoria and Yamagata B strains and mismatching with vaccine influenza B strains were found during 2005-2015 in Greece. The wide use of a QIV instead of a TIV will confer additional immunity and therefore protection against influenza B, and it is expected to prevent several SARI cases annually. Our findings strongly support the recommendations for using QIV.