Influenza B virus

The influenza B virus is a specific type of germ that belongs to the genus Betainfluenzavirus. It is part of a larger family of viruses called Orthomyxoviridae. This virus is classified as a negative-sense single-strand RNA virus. This means its genetic material is a single strand of RNA that must be converted into a positive sense before it can work. While some viruses infect many different animals, influenza B has a limited host range. It is known to infect humans, ferrets, pigs, and seals. Because it stays within these specific mammal species, it does not usually cause the massive global pandemics seen with influenza A. However, it still causes significant illness and death worldwide. It especially impacts adolescents and schoolchildren.

To understand how the virus works, we must look at its physical structure. The virus is an enveloped virus, meaning it is wrapped in an outer layer called an envelope. The actual particle, or virion, is quite complex. It contains a matrix protein, a nucleoprotein complex, a nucleocapsid, and a polymerase complex. The shape of the virus is not always the same. It can appear as a sphere or as a long, thin filament. On the surface of this envelope, there are approximately 500 projections. These projections are made of two important proteins: hemagglutinin (HA) and neuraminidase (NA). These surface antigens are essential for the virus to function and interact with host cells.

The genetic blueprint of the influenza B virus is also very organized. Its genome is 14,548 nucleotides long. This genome is not one long piece, but is instead divided into eight segments of linear, negative-sense, single-stranded RNA. This is known as a multipartite genome. Each of these eight segments is held inside its own separate nucleocapsid. These nucleocapsids are then all contained within a single outer envelope. Scientists believe that influenza A and B viruses diverged from a single common ancestor about 4,000 years ago. This is a much more recent split than the divergence between influenza A, B, and C, which happened about 8,000 years ago.

Scientists have studied the evolution of this virus through a process called antigenic drift. This occurs when small mutations happen in the genes for the surface proteins, hemagglutinin and neuraminidase. These tiny changes eventually create new strains that the human immune system might not recognize. Because of this constant shifting, influenza B viruses evolve more slowly than type A viruses, but faster than type C viruses. Specifically, influenza B mutates at a rate about two to three times slower than type A. This drift is the main reason why people can get the flu more than once and why vaccines must be updated every year to match the most current strains.

The history of discovering and fighting influenza B is quite detailed. In 1936, Thomas Francis Jr. discovered the influenza B virus in ferrets. That same year, Macfarlane Burnet discovered that the virus could be grown in fertilized hen eggs. This discovery was vital because it allowed researchers to study the virus and create inactivated vaccines in the late 1930s and early 1940s. By the 1950s, the effectiveness of these inactivated vaccines was proven. Later, in 2003, the first live, attenuated vaccine was approved for use. In 1942, scientists even developed a bivalent vaccine that protected against both the H1N1 strain of influenza A and the newly discovered influenza B.

For many years, influenza B was divided into two distinct lineages: B/Yamagata and B/Victoria. These lineages are distinguished by differences in the structure of their hemagglutinin proteins. They also trigger different types of innate immune responses in the people they infect. Until recently, quadrivalent vaccines were used to protect against four different viruses. These included two types of influenza A and both the Yamagata and Victoria lineages of influenza B. However, a surprising change occurred during the COVID-19 pandemic. Because of the measures taken to stop COVID-19, the B/Yamagata lineage may have become extinct. There have been no confirmed naturally occurring cases of B/Yamagata since March 2020.

This potential extinction has led to major changes in global health recommendations. In October 2023, the World Health Organization concluded that protection against the Yamagata lineage was no longer necessary. This decision allows the number of lineages targeted by seasonal vaccines to drop from four to three. For the 2024–2025 Northern Hemisphere season, both the US Food and Drug Administration and the European Medicines Agency have recommended removing B/Yamagata from all influenza vaccines. If the virus is indeed extinct, it would be the first documented case of a virus disappearing due to changes in human behavior. Even so, scientists continue to monitor the situation closely to ensure the remaining three strains are well-managed.