Influenza B virus is only known to infect certain mammal species, including humans, ferrets, pigs, and seals.[4][5] This limited host range is apparently responsible for the lack of influenzapandemics associated with influenza B virus, in contrast with those caused by the morphologically similar influenza A virus, as both mutate by both antigenic drift and reassortment.[6][7][8] Nevertheless, it is accepted that influenza B virus could cause significant morbidity and mortality worldwide, and significantly impacts adolescents and schoolchildren.[9]
There are two known circulating lineages of influenza B virus based on the antigenic properties of the surface glycoprotein hemagglutinin. The lineages are termed B/Yamagata/16/88-like and B/Victoria/2/87-like viruses.[10] The quadrivalent influenza vaccine licensed by the CDC has been designed to protect against both co-circulating lineages and as of 2016 has been shown to have greater effectiveness in prevention of influenza caused by influenza B virus than the previous trivalent vaccine.[11]
However, the B/Yamagata lineage might have become extinct in 2020/2021 due to COVID-19 pandemic measures.[12] In October 2023, the World Health Organization concluded that protection against the Yamagata lineage was no longer necessary in the seasonal flu vaccine, reducing the number of lineages targeted by the vaccine from four to three.[13][14] For the 2024–2025 Northern Hemisphere influenza season, the US Food and Drug Administration (FDA) recommends removing B/Yamagata from all influenza vaccines.[15] The European Medicines Agency (EMA) recommends removing B/Yamagata from influenza vaccines for the 2024–2025 seasonal flu vaccine composition.[16]
^Matsuzaki Y, Sugawara K, Takashita E, Muraki Y, Hongo S, Katsushima N, et al. (September 2004). "Genetic diversity of influenza B virus: the frequent reassortment and cocirculation of the genetically distinct reassortant viruses in a community". Journal of Medical Virology. 74 (1): 132–140. doi:10.1002/jmv.20156. PMID15258979. S2CID31146117.{{cite journal}}: CS1 maint: overridden setting (link)
^van de Sandt CE, Bodewes R, Rimmelzwaan GF, de Vries RD (September 2015). "Influenza B viruses: not to be discounted". Future Microbiology. 10 (9): 1447–1465. doi:10.2217/fmb.15.65. PMID26357957.