Influenza B virus (IBV) is only know to infect human beings which belongs to the family of Orthomyxovirus, and like with influenza A virus, IBV can cause human respiratory infections. This limited host and range is apparently responsible for the lack of Influenza virus B-caused influenza pandemics in contrast with those caused by the morphologically similar Influenza virus A as both mutate by both antigenic drift and reassortment. Currently there are two co-circulating lineages of the 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. The quadrivalent influenza vaccine licensed by the CDC is currently designed to protect against both co-circulating lineages and has been shown to have greater effectiveness in prevention of influenza caused by influenza B virus than the previous trivalent vaccine.
Influenza B virus possesses negative-sense, single-stranded, non-circle RNAs. The genome of it consists of eight independent fragments of which combines with ribonucleoprotein and form a RNA-protein complex. Each fragment contains a relatively conserved non-coding sequence and one open reading frame (ORF). The 13 nucleotides of the 5 'end of the gene were highly conserved and had the following sequence: 3'-GGAACAAAGAUGAppp-5', and also12 highly conserved nucleotides at the 3 'end: 3'-HO · UCGUCUuuCGUCC-5. The genome of influenza virus B contains 10 gene segments which separately encodes 10 proteins: PB1, PB2, PA, HA, 5-NP, NA, M and NS protein. Gene fragments 1 to 6 encode the influenza virus structural protein, and fragment 7 encodes the matrix protein. Fragment 8 encodes a nonstructural protein. Structural proteins are glycosylated and non-glycosylated points. Glycosylated structural proteins include HA and NA, the rest are non-glycosylated structural proteins. The glycosylation sites of HA and cleavage sites of mutations, may lead to a fundamental change in viral virulence. Figure 1 shows the schematic model of influenza B virus:
Fig. 1 the Schematic Model of Influenza B virus
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