• Seung Won Jung, Ye Jin Kim, Seung Beom Han, Kyung-Yil Lee, and Jin Han Kang.
• Department of Pediatrics, College of Medicine, The Catholic University of Korea , Seoul, Republic of Korea.
• Postgrad Med. 2021 Jan 1; 133 (1): 82-88.

ObjectivesSince the 2000s, two lineages of the influenza B virus (influenza B/Victoria and influenza B/Yamagata) have been co-circulating. Information on the age distribution of patients infected by each influenza B virus lineage may be helpful for establishing differentiated influenza prevention and control strategies for each age group.MethodsAge distributions were compared between patients infected by influenza A and B viruses and between those infected by the influenza B virus when B/Victoria and B/Yamagata lineages circulated dominantly.ResultsBetween the 2014-2015 and 2018-2019 influenza seasons, 2,718 and 1,397 patients were diagnosed with influenza A and B virus infections, respectively. The median age of patients infected by the influenza B virus was lower than that of patients infected by the influenza A virus (8 vs 12 years, p < 0.001). In the Yamagata season, the median ages of patients infected by influenza A and B viruses were similar (12 vs 11 years, p = 0.732); however, in the Victoria season, the median age of patients infected by the influenza B virus was lower than that of patients infected by the influenza A virus (6 vs 10 years, p < 0.001). In patients infected by the influenza B virus, patients aged <6 years and those aged ≥6 years were more likely to be infected during the Victoria and Yamagata seasons, respectively (p < 0.001).ConclusionThe age distribution of patients infected by the influenza virus was different between the Yamagata and Victoria seasons. Different influenza prevention and control strategies should be considered on the basis of the predominantly circulating virus and the affected age group.

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