Influenza B is a serious respiratory infection that can be just as dangerous as influenza A. Among hospitalized adults, a CDC study covering eight flu seasons found no significant difference between influenza A and B in ICU admission rates, length of hospital stay, or proportion of deaths. The perception that influenza B is “milder” is a common misunderstanding, likely fueled by the fact that it causes fewer total cases, not less severe ones.
How Influenza B Compares to Influenza A
The key difference between influenza A and B isn’t severity per case. It’s scale. A CDC study tracking over 24,000 flu-related hospitalizations between 2005 and 2013 found that influenza B accounted for roughly 3,500 of those admissions compared to about 21,000 for influenza A. But among the people who were hospitalized, both virus types produced equally severe outcomes. ICU admissions, hospital stays, and death rates were comparable regardless of which type a patient had.
Influenza A gets more attention because it’s responsible for pandemics and tends to dominate most flu seasons. Influenza B doesn’t have pandemic potential because it primarily infects humans and doesn’t jump between animal species the way influenza A does. But in any given season, influenza B can be the dominant circulating strain and drive a significant wave of illness.
Children Face the Highest Risk
Influenza B hits children especially hard. Kids under five are particularly vulnerable to severe complications, and influenza B accounts for a disproportionate share of pediatric flu deaths relative to its overall case numbers. Among 166 fatal pediatric cases involving flu-related brain inflammation reported to the CDC between the 2010-11 and 2024-25 seasons, 28% were caused by influenza B, a share that outweighs its typical proportion of total flu cases. The median age of these children was six years old.
This pattern holds for non-fatal illness too. Children with influenza B experience significant rates of hospitalization and complications, making it one of the most important infectious disease threats in pediatric populations during flu season.
Potential Complications
Most people with influenza B recover within one to two weeks with rest and fluids. But the virus can trigger life-threatening complications, particularly in young children, older adults, and people with weakened immune systems or chronic health conditions.
- Pneumonia is the most common serious complication, occurring when the virus damages the lungs directly or when bacteria take advantage of the weakened airways.
- Heart inflammation (myocarditis or pericarditis) can develop when the body’s immune response to the virus affects heart tissue, causing chest pain, rapid heartbeat, or in severe cases, heart failure.
- Brain inflammation (encephalopathy or encephalitis) is rare but particularly dangerous in children. The virus doesn’t typically infect the brain directly. Instead, the body’s inflammatory response becomes dysregulated and causes brain swelling and dysfunction. Acute necrotizing encephalopathy is one of the most severe forms and can be fatal.
How Long You’re Sick and Contagious
Symptoms of influenza B typically include fever, body aches, cough, sore throat, and fatigue. Most healthy adults feel significantly better within five to seven days, though cough and fatigue can linger for two weeks or more.
You become contagious about one day before symptoms appear, which is part of why flu spreads so efficiently. Viral shedding continues for five to seven days after you start feeling sick. The first three days of illness are the most contagious period. Young children and immunocompromised individuals can spread the virus for longer, sometimes beyond a week.
Only One Lineage Remains
Influenza B historically circulated as two distinct lineages: Victoria and Yamagata. This made vaccine planning complicated because manufacturers had to guess which lineage would dominate each season. That problem appears to be resolving itself. The Yamagata lineage has not been reliably detected in circulation since early 2020. A systematic review of global surveillance data found that the few reported Yamagata cases since then were mostly vaccine-derived strains, data entry errors, or unconfirmed detections.
This probable extinction means all circulating influenza B is now the Victoria lineage, which has prompted changes in vaccine formulation. Flu vaccines are shifting from quadrivalent (covering two A strains and two B lineages) to trivalent (two A strains and one B lineage), which could improve vaccine matching and simplify manufacturing.
How Well the Vaccine Works Against Influenza B
The flu vaccine tends to perform better against influenza B than influenza A. During the 2025-26 season, vaccine effectiveness against influenza B outpatient visits was 63% among adults and ranged from 45% to 71% among children and adolescents, depending on the surveillance network. For comparison, effectiveness against influenza A in adults was around 34% that same season.
The relatively strong performance against influenza B likely reflects the fact that with only one lineage now circulating, vaccine makers can achieve a closer match to the virus people actually encounter. Antiviral medications also work against influenza B, and early treatment within the first 48 hours of symptoms can shorten illness duration and reduce the risk of complications.