Flu B is just as severe as flu A. That’s the short answer, and it surprises most people. A CDC study covering eight flu seasons and nearly 25,000 hospitalizations found no significant difference between the two types in ICU admission rates, length of hospital stay, or deaths among hospitalized adults. The widespread belief that flu B is a “milder” version simply isn’t supported by the data.
Flu B vs. Flu A: How Severity Compares
The perception that flu B is less dangerous likely comes from the fact that it’s less common. During most flu seasons, influenza A causes the majority of infections, and flu B accounts for a smaller share. But less common does not mean less serious. Among people sick enough to be hospitalized, the outcomes are virtually identical regardless of type.
The CDC study, which analyzed over 21,000 flu A hospitalizations and 3,500 flu B hospitalizations from 2005 through 2013, found that flu B caused a similar proportion of deaths as flu A. ICU admissions were comparable season after season. In practical terms, if you’re lying in a hospital bed with the flu, it makes little difference whether the test comes back A or B.
Why Children May Face Extra Risks
In kids, the picture looks similar overall but with a few noteworthy patterns. A study of 391 hospitalized children found no significant differences in clinical outcomes, ICU treatment, or length of stay between flu A and flu B. About 14% of all the children in the study required intensive care, and roughly a quarter of those needed mechanical ventilation, highlighting that both types can hit hard.
Two complications appeared slightly more often with flu B in children, though neither reached statistical significance. Febrile convulsions (fever-triggered seizures) occurred in about 11% of children with flu B compared to 5% with flu A. Myositis, a painful inflammation of the muscles that can make it difficult for a child to walk, was diagnosed in roughly 4.5% of flu B cases versus 1% of flu A cases. These trends are worth knowing about as a parent, even though the overall severity between types was comparable.
Possible Complications
The most concerning complication of any flu infection is pneumonia. When the virus damages the airways, bacteria can move in and cause a secondary infection on top of the original illness. In a study of critically ill flu patients in ICUs, the most common bacteria behind these secondary pneumonias were Acinetobacter baumannii and Staphylococcus aureus. Patients who developed these secondary infections faced longer stays on ventilators, longer ICU admissions, and longer overall hospitalizations.
Other potential complications include inflammation of the heart muscle, inflammation of the brain, and worsening of chronic conditions like asthma, diabetes, or heart disease. These complications are not unique to flu B. They can happen with any influenza infection, and the risk goes up significantly for adults over 65, young children, pregnant women, and people with weakened immune systems.
Symptoms and Recovery Timeline
Flu B causes the same core symptoms as flu A: sudden fever, body aches, chills, cough, sore throat, fatigue, and sometimes vomiting or diarrhea (more common in children). Most healthy adults start to feel better within 5 to 7 days, though coughing and fatigue can linger for two weeks or more.
You’re contagious starting about one day before symptoms appear and for roughly five to seven days after getting sick. The first three days of illness are the most contagious period. Young children and people with weakened immune systems may spread the virus for longer, sometimes beyond that seven-day window.
Testing and False Negatives
If you get a rapid flu test at a clinic or urgent care, be aware that these tests are less sensitive for flu B than for flu A. The FDA requires rapid tests to detect flu B with at least 80% sensitivity, meaning up to 20% of true flu B infections could come back negative. For flu A, the sensitivity threshold is higher when compared to viral culture (90%). So a negative rapid test doesn’t rule out flu B, especially if your symptoms strongly suggest influenza. More accurate molecular tests (PCR) are available, though results take longer.
Which Flu B Strains Are Circulating
Influenza B has two main lineages: Victoria and Yamagata. As of 2024, only Victoria is still circulating. No Yamagata lineage viruses have been detected anywhere in the world since March 2020. This shift prompted the FDA to move from a four-component flu vaccine to a three-component (trivalent) vaccine for the 2024-25 and 2025-26 seasons, dropping the Yamagata component entirely.
During the 2024-25 season, flu B made up a small fraction of overall influenza detections, with 97% of positive specimens being influenza A. But flu B activity varies dramatically from year to year. Some seasons it accounts for a much larger share of infections, and when it does, the clinical impact is just as significant as any flu A-dominated season.
The Bottom Line on Severity
Flu B deserves the same respect as flu A. It causes the same range of symptoms, carries the same risk of hospitalization and complications, and kills at comparable rates among those who end up in the hospital. The annual flu vaccine covers the currently circulating B/Victoria lineage alongside flu A strains, and antiviral medications work against both types when started within the first 48 hours of symptoms.