Influenza B virus

Influenza B virus is the only species in the genus Betainfluenzavirus in the virus family Orthomyxoviridae.

Influenza B virus is only known to infect certain mammal species, including humans, ferrets, pigs 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. Nevertheless, it is accepted that Influenza B virus could cause significant morbidity and mortality worldwide, and significantly impacts adolescents and schoolchildren.

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 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.

However, the B/Yamagata lineage might have become extinct in 2020/2021 due to COVID-19 pandemic measures, 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.