What You Need to Know About Influenza B

Only a few strains of this type of influenza are included in the flu vaccine

Sick woman lying in bed
What Influenza B means for you. Sarah Kastner / STOCK4B / Getty Images

You may be surprised to learn that there are three main types of influenza: A, B, and C. Influenza A and B cause outbreaks of seasonal flu, but influenza C does not.

There is not much difference between influenza A and B when it comes to how they affect you, meaning one is not more or less severe than the other. 

That said, influenza A is the most common type of the virus and is further broken down into subtypes (for example, H1N1) and individual strains.

On the other hand, influenza B is not broken down into subtypes like influenza A is, but it is broken down into individual strains.

It's important to note that influenza B can cause outbreaks of seasonal flu, but they occur less frequently than outbreaks of influenza A. In addition, influenza A can cause pandemics of flu whereas influenza B cannot.

A flu pandemic refers to a global outbreak of a new influenza A virus that can spread efficiently and infect people easily. The influenza B virus does not form pandemics because it mutates slowly, which means it undergoes change gradually. In other words, influenza B cannot surprise a person's immune system with a brand new virus (like influenza A can).

Symptoms of Influenza B

Symptoms of influenza B are the same as the symptoms of other types of the flu and include:

  • Coughing
  • Fever
  • Chills
  • Sore throat and hoarseness
  • Headache
  • Body aches
  • Exhaustion
  • Congestion

    When you get the flu, the symptoms come on strong and fast, not gradually, and they typically last between three and seven days. Besides fever, extreme fatigue is a distinguishing symptom between an infection with the influenza virus and another virus that causes the common cold (for example, rhinovirus) —a person with the flu feels so bad that it's hard to do almost anything other than lay in bed.

    Diagnosis of Influenza B 

    If you think you might have the flu, see your health care provider within the first 48 hours of the onset of your symptoms. He or she can perform a flu test and diagnose you based on the test results, your symptoms, and a physical exam.

    If your health care provider diagnoses you with the flu, you may be able to take antiviral medications to shorten the duration of your illness and lessen the severity of your symptoms.

    It's worthy to note that some flu tests can identify whether you have influenza A or B, but many rapid tests that are performed in the doctor's office do not; they only tell you whether or not you have the flu.

    Unfortunately, even those tests are not always accurate, which is why a doctor's clinical diagnosis is critical, meaning his or her assessment of your symptoms, vitals, and physical examination. 


    Because influenza B is less common but can still cause outbreaks of seasonal flu, only one or two strains of influenza B are included in the seasonal flu vaccine every year to protect people—it's the strain(s) that researchers believe are most likely to cause illness during the upcoming flu season.

    More specifically, the quadrivalent flu vaccine contains two strains of influenza B (and two strains of influenza A), but the traditional trivalent flu vaccine only contains one of influenza B (and two of influenza A).

    The nasal spray vaccine (called FluMist) also has four strains, but it contains live attenuated influenza virus, unlike the flu shot. The high dose vaccine (called Fluzone) has three strains but it contains four times as much of the flu antigen, which is why it's commonly offered to older people who require more antigen to develop a proper immune system response.

    In the end, the most important thing is to get vaccinated. Do not wait for a specific vaccine and risk your chances of getting the flu in the process. However, be sure to talk to your doctor if you are allergic to egg proteins, had a reaction to the influenza vaccine in the past, are currently ill, or had Guillain-Barre syndrome within six weeks of getting the flu vaccine.

    A Word From Verywell

    If you come down with the flu, whether it is influenza A or B, take care of yourself, get plenty of rest, drink lots of fluids, and stay away from other people so you don't spread the virus.

    If you are at high risk for complications from the flu or you believe you may have developed a secondary infection, be sure to see your healthcare provider right away.


    Centers for Disease Control and Prevention. (2016). Frequently Asked Flu Questions 2016-2017 Influenza Season.

    C enters for Disease Control and Prevention. (2016). Types of Influenza Viruses.

    Glezen WP, Schmier JK, Kuehn CM, Ryan KJ, Oxford J. The burden of Influenza B: A structured literature review. Am J Public Health. 2013 March;103(3):e43-e51.

    US National Library of Medicine. (2016). PubMed Health. Flu: Overview.

    Continue Reading