Influenza B virus

This tiny germ can make people sick. It mostly lives in humans. It can also live in pigs and seals.

This germ changes a little bit every year. These small changes help it stay alive. Because it changes, we need new shots each year.

One kind of this germ may be gone. It might have gone away because of how people stayed safe during COVID-19. Doctors are watching to see if it stays gone. It is very interesting to learn about germs!

The influenza B virus is a tiny germ. It mostly lives in humans. It can also live in pigs and seals.

This virus has a special shape. It can be round or long like a thread. It has many small parts on its surface. These parts are called hemagglutinin and neuraminidase. They help the virus work.

The virus changes in a way called antigenic drift. This means it makes small changes to its parts. These changes help it hide from our bodies. Because it changes, we need new flu shots each year.

For a long time, there were two main groups. We call these the Yamagata and Victoria lineages. One group, Yamagata, might be gone now. It may have disappeared because of COVID-19 rules. No new cases were found after March 2020.

Doctors now make vaccines with fewer parts. They do not need to protect against Yamagata anymore. They still use vaccines to help us stay safe from other flu types.

The influenza B virus is a tiny germ that affects living things. It mostly lives in humans, but it can also infect pigs, seals, and ferrets. Because it stays in these specific animals, it does not cause huge global pandemics like influenza A does. However, it can still make many people quite sick. It often impacts schoolchildren and teenagers the most. Scientists study it to understand how it moves through different groups of people.

This virus has a very specific way it works. Its body is made of an envelope and several proteins. It can look like a round ball or a long thread. On its surface, it has about 500 little projections. These are called hemagglutinin and neuraminidase. These parts are very important for the virus. The virus also has a genome made of eight segments of RNA. This genetic material is what tells the virus how to act.

People have been learning about this virus for a long time. In 1936, Thomas Francis Jr. discovered the influenza B virus in ferrets. That same year, Macfarlane Burnet found that viruses could grow in hen eggs. This helped scientists start making vaccines in the late 1930s and early 1940s. By the 1950s, these vaccines were proven to work well. Later, in 2003, the first live, weakened vaccine was approved.

There are two main groups of this virus called lineages. We call them the B/Yamagata and B/Victoria lineages. These two groups are different in how they look on the outside. They also trigger different responses in our immune systems. For a long time, vaccines protected against both groups. However, the Yamagata group might be extinct now. No natural cases were found after March 2020. This might have happened because of rules used during the COVID-19 pandemic.

Because the virus changes, we must update our protection. It uses a process called antigenic drift to make small changes. These small changes help the virus hide from our bodies. This is why doctors change the flu vaccine every year. In October 2023, the World Health Organization said we no longer need to target the Yamagata lineage. For the 2024–2025 season, the FDA and EMA recommend removing it from vaccines. This reduces the number of targets in the vaccine from four down to three.

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.