Influenza B is one of two main types of flu virus that cause seasonal outbreaks in humans every year. Unlike influenza A, which circulates in birds, pigs, and other animals, influenza B infects almost exclusively humans (and, rarely, seals). This narrow host range is the key reason influenza B never causes pandemics, but it still drives significant illness, hospitalization, and death during regular flu seasons.
How Influenza B Differs From Influenza A
Both influenza A and B belong to the same virus family (Orthomyxoviridae) and share a similar structure: an envelope surrounding eight segments of genetic material that code for 11 proteins. The virus particles are tiny, roughly 80 to 120 nanometers across, and can be round or filament-shaped. Two proteins on the surface do most of the work. One helps the virus latch onto cells in your respiratory tract and get pulled inside. The other acts like a pair of molecular scissors, snipping the virus free from the cell surface so newly made copies can spread to neighboring cells.
The critical difference is in where these viruses live. Influenza A circulates widely in birds and mammals, and when strains from different animal hosts swap genetic segments, entirely new versions of the virus can emerge. That process is what sparks pandemics. Influenza B, limited to humans and seals, has far fewer opportunities to reshuffle its genes in dramatic ways. It still mutates over time and produces new seasonal strains, but the changes are more gradual.
For decades, influenza B split into two distinct lineages called Victoria and Yamagata. Both circulated simultaneously and complicated vaccine planning each year. That changed recently: the Yamagata lineage appears to have disappeared from circulation. The World Health Organization now recommends that trivalent flu vaccines include only a Victoria lineage strain, and has advised that quadrivalent vaccines should exclude the Yamagata component since it is no longer warranted.
Symptoms and Severity
If you have influenza B, the symptoms are essentially the same as influenza A: fever, body aches, chills, cough, sore throat, fatigue, and sometimes vomiting or diarrhea (especially in children). There is no reliable way to tell the two types apart based on how you feel. The only way to confirm which type you have is a lab test.
Studies comparing hospitalized children with influenza A and B have found no significant differences in severity, length of hospital stay, need for intensive care, or clinical outcomes. In infants under six months, both types can present as a sepsis-like illness with high fever and poor feeding, which often leads to broad testing and antibiotic treatment until results come back. The bottom line: influenza B is not a “milder” version of the flu. It carries the same potential for serious complications.
How It Spreads and How Long You’re Contagious
Influenza B spreads the same way influenza A does: through respiratory droplets when an infected person coughs, sneezes, or talks, and sometimes by touching contaminated surfaces and then touching your eyes, nose, or mouth. Symptoms typically appear about two days after exposure, though the window ranges from one to four days.
You can start spreading the virus to others a full day before your symptoms appear, which is part of what makes flu so hard to contain. Once you’re sick, you remain contagious for roughly five to seven days. The first three days of illness are when you’re most likely to pass it on. Young children and people with weakened immune systems may shed the virus for longer.
Complications to Watch For
Most healthy adults recover from influenza B within one to two weeks, but the virus can trigger serious complications at any age. Pneumonia is the most well-known, but the list also includes bacterial co-infections, seizures, and inflammation of the brain.
Children face particular risks. During the 2024-25 flu season, the CDC reported 280 pediatric flu deaths in the United States, a rate of 3.8 deaths per million children. Among those children, 56% had at least one underlying medical condition, meaning 44% were previously healthy. The most common complications before death were shock or sepsis (50%), pneumonia (38%), acute respiratory distress syndrome (28%), seizures (24%), and brain inflammation (18%). When children were tested for bacterial infections, 41% had a dangerous pathogen isolated, most commonly Staphylococcus aureus, Streptococcus pneumoniae, or group A Streptococcus.
One striking detail: 22% of pediatric deaths occurred outside a hospital, and another 27% happened in the emergency department before the child could be admitted. This underscores how quickly flu complications can escalate, particularly in young children.
Treatment With Antivirals
The same antiviral medications used for influenza A also work against influenza B. The most commonly prescribed option is oseltamivir (Tamiflu), taken as a pill or liquid twice daily for five days. Two other drugs in the same class are available as an inhaler or an IV infusion. All three work by blocking the surface protein that helps newly made virus particles escape from infected cells, slowing the spread of the virus in your body.
A newer antiviral, baloxavir (Xofluza), works through a completely different mechanism and is taken as a single dose. In a randomized trial, baloxavir reduced symptom duration in adolescents and adults with influenza B by more than 24 hours compared to oseltamivir. That makes it a particularly strong option when influenza B is confirmed or suspected, though both medications are effective.
Antivirals work best when started within 48 hours of symptom onset. They don’t cure the flu instantly, but they can shorten illness by one to two days and, more importantly, reduce the risk of serious complications.
How Vaccines Address Influenza B
Every seasonal flu vaccine includes at least one influenza B strain. For the 2024-25 Northern Hemisphere season, the WHO recommended a B/Victoria lineage component in all flu vaccines. This is a shift from previous years, when quadrivalent vaccines included strains from both the Victoria and Yamagata lineages. With Yamagata no longer circulating, the trend is moving back toward trivalent vaccines that cover two influenza A strains and one influenza B strain.
Because influenza B mutates more slowly than influenza A, vaccine matches for the B component tend to be more reliable from year to year. That said, the virus still evolves enough to require updated strains periodically, which is why the WHO reviews and revises its vaccine recommendations twice a year for the Northern and Southern Hemispheres.