Influenza B virus is the only species in the genus Betainfluenzavirus in the virus family Orthomyxoviridae. Influenza B virus is known only to infect humans and seals. This limited host range is apparently responsible for the lack of associated influenza pandemics in contrast with those caused by the morphologically similar influenza A virus as both mutate by both antigenic drift and reassortment. 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. 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. Further diminishing the impact of this virus, "in humans, influenza B viruses evolve slower than A viruses and faster than C viruses". Influenzavirus B mutates at a rate 2 to 3 times slower than type A. Nevertheless, it is accepted that Influenza B virus could cause significant morbidity and mortality worldwide, and significantly impacts adolescents and schoolchildren. The B/Yamagata lineage might have become extinct in 2020/2021 due to COVID-19 pandemic measures. Morphology The Influenza B virus capsid is enveloped while its virion consists of an envelope, a matrix protein, a nucleoprotein complex, a nucleocapsid, and a polymerase complex. It is sometimes spherical and sometimes filamentous. Its 500 or so surface projections are made of hemagglutinin and neuraminidase. Genome structure and genetics The Influenza B virus genome is 14,548 nucleotides long and consists of eight segments of linear negative-sense, single-stranded RNA. The multipartite genome is encapsidated, each segment in a separate nucleocapsid, and the nucleocapsids are surrounded by one envelope. The subtypes of influenza A virus are estimated to have diverged 2,000 years ago. Influenza viruses A and B are estimated to have diverged from a single ancestor around 4,000 years ago, while the ancestor of influenza viruses A and B and the ancestor of influenza virus C are estimated to have diverged from a common ancestor around 8,000 years ago. Metatranscriptomics studies have also identified closely related "Influenza B-like" viruses such as the Wuhan spiny eel influenza virus and also "Influenza-B like" viruses in a number of vertebrate species such as salamanders and fish. Vaccine In 1936, Thomas Francis Jr. discovered the ferret influenza B virus. Also in 1936, Macfarlane Burnet