Influenza B: What is It, Causes, Symptoms, Treatment & Prevention 

Introduction

Influenza B is one of the primary types of influenza viruses responsible for seasonal flu outbreaks across the globe¹. Among the estimated 1 billion cases of influenza that occur annually, influenza B contributes significantly, more commonly in children and adolescents, however, it can affect all age groups^1,2. 

Although less prevalent than influenza A, influenza B virus can co-circulate with it and have varying global impact based on season and region³. Therefore, understanding what influenza B is, its symptoms and its management strategies is essential for effective prevention and treatment, especially during peak flu seasons. 

What Causes Influenza B?

Influenza B is caused by the influenza B virus, a member of the Orthomyxoviridae family⁴. This virus is divided into two genetically and antigenically distinct lineages⁴:  

• B/Victoria/2/87-like or the Victoria lineage⁴
• B/Yamagata/16/88-like or the Yamagata lineage (this lineage has not been detected since March 2020 and is expected to go extinct)^4,5 

The influenza B virus spreads easily from person to person through respiratory droplets released when an infected individual coughs or sneezes, making close contact a key factor in transmission. It can also spread by touching contaminated surfaces and then touching your face without washing your hands². 

While influenza B typically leads to mild to moderate respiratory illness, particularly in otherwise healthy children, it can sometimes result in more severe disease. This is especially true for individuals with underlying chronic medical conditions, weakened immune systems, or other health vulnerabilities. In rare cases, it can even be fatal⁴.

Influenza B Symptoms

Influenza B symptoms usually begin within 1 to 4 days after infection and can include^2,6: 

• Sudden onset of high fever 
• Dry, persistent cough (may last 2 weeks or more) 
• Headache 
• Muscle and joint pain 
• Severe fatigue or general feeling of unwellness (malaise) 
• Sore throat 
• Runny or congested nose 

Note: Children (especially those under 5 years of age) may experience vomiting, abdominal pain and diarrhoea. However, these symptoms are not prevalent in adults⁷. 

How Influenza B Differs from Types A and C

Influenza B differs from types A and C in several important ways, including its virus structure, who it primarily affects, and the severity of the illness it causes: 

Virus Strains: 

• Influenza A has many subtypes (e.g., H1N1, H3N2). 
• Influenza B has two main lineages (B/Victoria and B/Yamagata) 
• Influenza C has no subtypes or lineages³. 

Who It Affects: 

• Influenza A infects humans and animals. 
• Influenza B and C infect only humans⁸. 

Severity: 

• Influenza A usually causes more severe illness. 
• Influenza B tends to be milder but can still be serious. 
• Influenza C causes mild, cold-like symptoms². 

Outbreaks: 

• Influenza A (particularly certain strains like A(H1N1)) can cause seasonal epidemics and pandemics.  
• Influenza B causes seasonal flu but not pandemics. 
• Influenza C rarely causes outbreaks².

Who is At Higher Risk of Influenza B

While influenza B can affect anybody, certain groups are at a greater risk of complications. These include: 

• Young children (especially under 5 years): Their immune systems are still developing, making them more susceptible⁹. 
• Older adults (65 years and above): Aging weakens the immune system, reducing the body’s ability to fight off infections². 
• People with chronic conditions: Individuals with asthma, diabetes, heart disease, kidney problems, or lung disorders may face more severe illness^2,9. 
• Those with weakened immune systems: People undergoing treatments like chemotherapy or those living with conditions like HIV/AIDS are at greater risk⁹. 
• Pregnant women: Changes in the immune system during pregnancy increase the risk of flu-related complications^2,9. 
• Healthcare workers and caregivers: Due to frequent contact with infected individuals, they have a higher chance of exposure and transmission, although not a higher risk of complications as such².

Diagnosis of Influenza B

Diagnosing influenza B typically starts with a clinical evaluation, where doctors review the patient’s symptoms and medical history¹⁰. To confirm the infection, several diagnostic tests may be used but the common ones are: 

• Rapid influenza diagnostic tests (RIDTs): These detect flu virus antigens (proteins) from a nose or throat swab. They can easily be done using test kits and give quick results but are less accurate, especially for Influenza B. A negative result doesn’t always mean that you’re flu-free, so further testing may be needed if there is strong clinical suspicion^11,12.^ 
• Molecular-Based Tests: These tests detect the virus’s genetic material (RNA) and are more accurate than antigen-based tests. They can also distinguish between influenza A and B viruses. Tests like reverse transcription polymerase chain reaction (RT-PCR) are usually done in a laboratory and are considered the most reliable. Moreover, rapid molecular assays are also available that provide quick and accurate results^11,12.
• Viral Culture: The virus is grown in a laboratory for detailed study. Although this is a highly accurate test, due to its slow processing time, this is rarely used for immediate diagnosis¹⁰. 

Other laboratory tests include immunofluorescence and serological assays to detect virus-specific antigens/antibodies. However, molecular tests have largely replaced these now¹¹. 

Complications

High-risk individuals should be especially vigilant for influenza B complications such as¹³Â 

• Pneumonia (viral or bacterial) 
• Sinus and ear infections 
• Myocarditis (heart inflammation) 
• Encephalitis (brain inflammation) 
• Myositis or rhabdomyolysis (muscle inflammation or breakdown) 
• Sepsis (a life-threatening response to infection) 
• Worsening of existing medical conditions 

Influenza B Treatment Options

Most cases of influenza B can be managed at home with supportive care, but antiviral medications may be recommended in some situations, especially for those at high risk of complications. Common influenza B treatment options include: 

• Antiviral Medications: These drugs can help reduce the severity and duration of influenza B symptoms, especially when started within 48 hours of symptom onset. They are especially recommended for all hospitalised patients, those with severe or progressive illness, and high-risk individuals (e.g., elderly, pregnant women, people with chronic conditions, etc.), regardless of how long symptoms have been present^10,12. 
• Rest: Adequate rest helps the body recover and conserve energy to fight the infection². 
• Hydration: Drinking plenty of fluids prevents dehydration, especially if fever is present².
• Over-the-Counter Pain Relievers: When taken under medical supervision, these can help relieve fever, headache, and body aches¹⁴. 

Prevention Tips

To protect yourself and those around you, it’s important to follow these effective prevention measures: 

• Get vaccinated annually: The flu vaccine (quadrivalent influenza vaccine offers broader protection by covering both lineages) is the most effective way to reduce your risk of influenza B infection and its complications^2,5. 
• Practice good hand hygiene: Wash your hands regularly with soap and water (for at least 20 seconds) or use an alcohol-based hand sanitiser. 
• Avoid close contact: Stay away from people who are sick, and if you’re unwell, keep your distance to prevent spreading the virus¹⁵. 
• Cover coughs and sneezes: Use a tissue (or your elbow) to cover your mouth and nose when coughing or sneezing. Wearing a mask will also help prevent the spread of infection¹⁵. 
• Clean and disinfect surfaces: Frequently clean high-touch surfaces like door handles, mobile phones, and countertops to lower the risk of contamination¹⁵.
• Maintain a healthy lifestyle: Support your immune system by getting enough sleep, eating a balanced diet, staying hydrated, managing stress, and staying physically active¹⁵.

When to See a Doctor

Most individuals recover from influenza B within 5 to 14 days, though symptoms like cough may persist for longer than 14 days^2,16.  

However, in some cases, symptoms may worsen or lead to serious complications. It’s important to consult a doctor immediately if you experience any of the following¹⁶: 

• Worsening of existing symptoms 
• Difficulty breathing or shortness of breath 
• Chest pain or pressure 
• Sudden dizziness, confusion, or disorientation 
• Persistent high fever that doesn’t improve 
• Bluish lips/face  
• Severe muscle pain, extreme weakness, or trouble walking 
• Little or no urine output 
• Flare-up of chronic health conditions (such as asthma or heart disease) 

Also Read: Influenza A: What is It, Causes, Symptoms, Treatment & Prevention

Conclusion

Understanding and effectively managing influenza B is crucial for reducing the spread of infection and minimising the risk of complications, especially among vulnerable groups like children, elderly, and those with weakened immune systems.  

While often milder than influenza A, influenza B can still lead to a serious illness. Timely diagnosis, appropriate influenza B virus treatment (such as antiviral therapy), preventive measures like vaccination, and awareness of warning signs, all play a vital role in controlling the impact of this seasonal virus. Thus, by staying informed and proactive, individuals and communities can better protect themselves and others during flu season.  

Frequently Asked Questions (FAQs)

References

1. Ashraf MA, Raza MA, Amjad MN, Ud Din G, Yue L, Shen B, Chen L, Dong W, Xu H, Hu Y. A comprehensive review of influenza B virus, its biological and clinical aspects. Front Microbiol. 2024 Sep 4;15:1467029. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC11408344/ 
2. World Health Organization. Influenza (seasonal) [Internet]. World Health Organization; [cited 2025 May 28]. Available from: https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal) 
3. Centers for Disease Control and Prevention. Types of Influenza Viruses [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/about/viruses-types.html 
4. Bhat YR. Influenza B infections in children: A review. World J Clin Pediatr. 2020 Nov 19;9(3):44-52. Available from: https://pubmed.ncbi.nlm.nih.gov/33442534/ 
5. Paget J, Caini S, Del Riccio M, van Waarden W, Meijer A. Has influenza B/Yamagata become extinct and what implications might this have for quadrivalent influenza vaccines? Euro Surveill. 2022 Sep;27(39):2200753. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9524051/ 
6. Boktor SW, Hafner JW. Influenza. StatPearls [Internet]. StatPearls Publishing; 2025 Jan; Available from: https://www.ncbi.nlm.nih.gov/books/NBK459363/ 
7. Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016 Dec 7;355:i6258. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5141587/ 
8. Liang Y. Pathogenicity and virulence of influenza. Virulence. 2023 Dec;14(1):2223057. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC10283447/ 
9. Coleman BL, Fadel SA, Fitzpatrick T, Thomas SM. Risk factors for serious outcomes associated with influenza illness in high- versus low- and middle-income countries: Systematic literature review and meta-analysis. Influenza Other Respir Viruses. 2018 Jan;12(1):22-29. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5818335/ 
10. Gaitonde DY, Moore FC, Morgan MK. Influenza: Diagnosis and Treatment. Am Fam Physician. 2019 Dec 15;100(12):751-758. Available from: https://pubmed.ncbi.nlm.nih.gov/31845781/ 
11. Vemula SV, Zhao J, Liu J, Wang X, Biswas S, Hewlett I. Current Approaches for Diagnosis of Influenza Virus Infections in Humans. Viruses. 2016 Apr 12;8(4):96. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4848591/ 
12. Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016 Dec 7;355:i6258. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5141587/ 
13. Moghadami M. A Narrative Review of Influenza: A Seasonal and Pandemic Disease. Iran J Med Sci. 2017 Jan;42(1):2-13. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC5337761/ 
14. InformedHealth.org [Internet]. Cologne (Germany): Institute for Quality and Efficiency in Health Care (IQWiG). Overview: Influenza [updated 2022 Nov 22; cited 2025 Jun 11]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279459/ 
15. Centers for Disease Control and Prevention. Healthy Habits to Prevent Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/prevention/actions-prevent-flu.html 
16. Centers for Disease Control and Prevention. Signs and Symptoms of Flu [Internet]. Centers for Disease Control and Prevention; [cited 2025 May 28]. Available from: https://www.cdc.gov/flu/signs-symptoms/index.html 
17. Saha S, Chadha M, Shu Y; Group of Asian Researchers on Influenza (GARI). Divergent seasonal patterns of influenza types A and B across latitude gradient in Tropical Asia. Influenza Other Respir Viruses. 2016 May;10(3):176-84. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC4814861/ 

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