To analyze the prevalence of influenza B virus in Hangzhou between 2014 and 2019, and the variation in hemagglutinin (HA) and neuraminidase (NA) genes.

Viruses were isolated from throat swabs collected from 10 481 patients with influenza-like illness in Hangzhou from October 2014 to September 2019. The subtypes of influenza viruses were identified by real-time RT-PCR. HA and NA genes of some influenza B virus strains were amplified with specific primers and then analyzed with gene sequencing and phylogenetic analysis.

Influenza B virus had circulated in Hangzhou every year since 2014 and caused a much higher morbidity in people aged 5-14 years. The Victoria lineage of influenza B virus circulating in Hangzhou between 2014 and 2019 belonged to the V1A clade, while the Yamagata lineage belonged to the Y3 clade. Homology analysis showed that 124 strains shared 94.67%-100.00% homology in HA gene sequences and 128 strains shared 97.13%-100.00% homology in NA gene sequences. Influenza B virus possessed several mutations in the antigenic sites of HA and NA.

Influenza B virus played a critical role in the influenza epidemics in Hangzhou. During 2014 to 2019, genetic mutations in the antigenic sites of HA and NA and reassortant strains were detected, and there was a mismatch between epidemic and vaccine strains. However, no drug-resistant virus was found.

Influenza B virus; Hemagglutinin; Neuraminidase; Genetic evolution