Further diminishing the impact of this virus "in man, influenza B viruses evolve slower than A viruses and faster than C viruses". Influenzavirus B mutates at a rate 2 to 3 times lower than type A. However, influenza B mutates enough that lasting immunity is not possible. For example at the U.S.'s Food and Drug Administration's (FDA) Center for Biologics Evaluation and Research's Vaccines and Related Biological Products Advisory Committee's 101st meeting of February 16, 2005, an extensive discussion and vote was held concerning next year's flu vaccine virus selection including which influenza B strain to use in the formulation of the flu vaccine:
"For Influenza B, the question was asked: are there new strains present? And the answer was yes, and in 2004, the majority of the viruses were similar to a strain called B/Shanghai/361/2002, which is from the so-called B/Yamagata/16/88 hemagglutinin lineage. That lineage was not the one that was being used in the vaccine that was current last year. In a minority of the strains that were found during the epidemiological studies were similar to the strain that was in the vaccine for last year [stet], which was B/Hong Kong/330/2001, which belongs to the HA lineage that we represent with the strain B/Victoria/287. In answer to the question were these new viruses spreading, the answer, of course, is definitely yes. The Fujian-like viruses had become widespread around the world and were predominant everywhere, and these B/Shanghai-like strains at the time we were holding this meeting in February were predominant not only in North America and the United States, but also in Asia and Europe."
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.
The Influenza B virus genome is 14548 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.
^Matsuzaki Y, Sugawara K, Takashita E, Muraki Y, Hongo S, Katsushima N, Mizuta K, Nishimura H (2004). "Genetic diversity of influenza B virus: the frequent reassortment and cocirculation of the genetically distinct reassortant viruses in a community". J. Med. Virol.74 (1): 132–40. doi:10.1002/jmv.20156. PMID15258979.
^Yamashita M, Krystal M, Fitch WM, Palese P (1988). "Influenza B virus evolution: co-circulating lineages and comparison of evolutionary pattern with those of influenza A and C viruses". Virology163 (1): 112–22. doi:10.1016/0042-6822(88)90238-3. PMID3267218.