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Influenza B viral infection is of great importance, but the epidemiological and phylogenetic characteristics of influenza B infection in severe acute respiratory infection (SARI) cases are still unclear.The clinical information of 2816 SARI cases and 467,737 influenza-like illness (ILI) cases in Beijing area from September 2014 to April 2015 were collected and analyzed. Among them, 91 influenza B viruses isolated from SARI cases were sequenced.The overall yield rate of influenza A/B infection was 14.21% and 27.77% in sampled SARI and ILI cases, respectively. Compared with influenza A infection, the frequency of influenza B infection in SARI cases was higher in younger patients. Phylogenetic analysis suggested that most tested hemagglutination genes belonged to Yamagata lineage Clade 3, which were similar with current circulating viruses but different with 2014 to 2015 influenza season vaccine strain (Clade 2). Importantly, HA-Y3/NA-V4 intralineage reassorting was identified in Beijing area for the first time, which can act as a possible risk factor of SARIs.The influenza activity and virus types/subtypes/lineages among SARI patients were well correlated with that of ILI cases. Furthermore, the potential risk of reassorted influenza B virus infection should not be overlooked.