Significant circulation of influenza B viruses mismatching the recommended vaccine-lineage in South Korea, 2007–2014

We aimed to characterize the lineages of influenza B viruses obtained from clinical specimens during the 2007–2014 seasons in South Korea. RT-PCR for the partial hemagglutinin gene of influenza B virus was performed on laboratory-confirmed influenza B samples from the 2007–2008 season to 2013–2014 season. A phylogenetic tree was generated, and current influenza vaccine strains for the Northern Hemisphere were used as representative strains of Victoria and Yamagata lineages. A total of 571 influenza B virus sequences were analyzed. During the 2009–2010 season, most of the circulating influenza B viruses matched the vaccine strain; 91.0% (91/100) of viruses belonged to the Victoria lineage. In the 2007–2008, 2011–2012, and 2013–2014 seasons, co-circulation of each influenza B lineage was found with a match ratio to the vaccine strain of 53.2% (42/79), 40.9% (63/154), and 58.3% (134/230), respectively. Overall, 41.7% (238/571) of the circulating influenza B viruses belonged to the lineage mismatching the vaccine strain. During the seven influenza seasons, influenza B epidemics were substantial in four seasons in South Korea. Significant mismatches of the vaccine and lineage of the circulating influenza B viruses were found. The current trivalent influenza vaccine may not be fully suitable for effective protection against influenza B.