Influenza B is generally considered to be a serious seasonal respiratory virus that circulates almost exclusively in humans. Unlike influenza A, which jumps between birds, pigs, and people and causes pandemics, influenza B has a much narrower host range and evolves more slowly. That distinction sometimes leads people to assume it’s the “milder” flu, but that reputation is misleading. Among hospitalized adults, influenza B causes equally severe outcomes as influenza A, including comparable rates of ICU admission and death.
How Influenza B Differs From Influenza A
The most important distinction is about host range and pandemic risk. Influenza A infects birds, pigs, and other animals, which gives it opportunities to swap genetic material across species and produce entirely new strains. That’s how pandemics start. Every flu pandemic on record, from 1918 onward, has been caused by influenza A.
Influenza B, by contrast, circulates almost entirely among humans. Seals are the only other known natural host, and transmission from seals to people remains questionable. Because the virus doesn’t cycle through animal reservoirs, it lacks the mechanism to undergo the dramatic genetic shifts that create pandemic strains. This makes influenza B a seasonal threat rather than a pandemic one, but within any given flu season, it can be the dominant circulating virus and cause widespread illness.
The Two Lineages: Victoria and Yamagata
Influenza B split into two distinct lineages in the late 1980s, named after the places where they were first characterized: B/Victoria (from a 1987 sample) and B/Yamagata (from 1988). These lineages differ in which surface proteins they display, which means immunity to one doesn’t reliably protect against the other. For decades, predicting which lineage would dominate each season was a guessing game that complicated vaccine design.
The two lineages also tend to hit different age groups. Victoria viruses show higher infection rates in children, while Yamagata viruses more commonly infect older adults. Victoria has also been the more genetically restless of the two, with frequent mutations in its surface proteins since 2010 that help it dodge the immune system.
In a significant development, the Yamagata lineage appears to have gone extinct. There have been no confirmed detections of circulating Yamagata viruses anywhere in the world since March 2020. The FDA concluded that Yamagata no longer poses a public health threat and recommended removing it from vaccines for the 2024-2025 season. Flu vaccines in the U.S. have shifted from four-strain (quadrivalent) formulations back to three-strain (trivalent) ones: two influenza A components and a single influenza B/Victoria component.
Severity and Symptoms
Influenza B produces the classic flu picture: fever, chills, headache, fatigue, cough, and sometimes vomiting or diarrhea. In children, it can also cause ear infections, croup, bronchiolitis, and pneumonia. The symptom profile is essentially indistinguishable from influenza A without a lab test.
A large CDC study covering eight flu seasons (2005 through 2013) compared over 21,000 influenza A hospitalizations with roughly 3,500 influenza B hospitalizations in adults. The results were clear: length of hospital stay, ICU admission rates, and the proportion of deaths were comparable between the two types. Influenza B is not a lesser illness. The reason it accounts for fewer total hospitalizations is simply that it circulates less frequently than influenza A in most seasons, not because it’s inherently gentler.
Children and adolescents aged 5 to 17 are disproportionately affected. During the 2012-13 and 2019-20 flu seasons, influenza B accounted for over half of all flu-related hospitalizations in that age group.
Complications to Watch For
One complication particularly associated with influenza B is myositis, an inflammation of the muscles that causes severe muscle pain, most often in the calves. In children, this can progress to rhabdomyolysis, where damaged muscle tissue releases proteins that can injure the kidneys. Myocarditis (inflammation of the heart muscle) is a rarer but more dangerous complication. Pediatric mortality from influenza B overall is under 5%, but when acute myocarditis develops, mortality climbs to around 7%.
Transmission and Contagiousness
The incubation period for influenza, including type B, is about two days after exposure, though it can range from one to four days. You’re most contagious during the first three days of symptoms. In healthy adults, the window of contagiousness can start a full day before symptoms appear and extend five to seven days after getting sick. Young children and people with weakened immune systems may spread the virus for even longer.
This pre-symptomatic transmission window is part of what makes flu difficult to contain. You can pass the virus to others before you even realize you’re sick, which is one reason seasonal flu remains so persistent despite widespread vaccination efforts.
What This Means for Vaccines
For years, vaccine manufacturers included both B/Victoria and B/Yamagata in their formulations to cover both lineages. With Yamagata’s apparent disappearance, the simpler trivalent vaccine is now standard. This shift has a practical benefit: removing a strain that no longer circulates eliminates the small but real risk of reintroducing Yamagata through live-attenuated vaccine viruses.
Because B/Victoria continues to mutate frequently, with insertions and deletions in its surface proteins that help it evade existing immunity, the Victoria component in each year’s vaccine is updated regularly. Getting vaccinated each season remains the most effective protection, since the circulating version of Victoria can look quite different from one year to the next.