Flu A vs Flu B: What Are the Differences?

Does it matter if I have flu A or flu B?

There are four types of influenza viruses, designated A, B, C, and D, but the most common in humans are A and B. Though the two have similar symptoms and are treated in the same way, there are differences between them, including how serious they can become in different populations.

This article will discuss the similarities and differences in influenza type A and type B, including who it affects, severity, symptoms, causes, diagnosis, treatment, and prevention.

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What Are the Main Differences Between Flu A and Flu B?

Differences between flu A and flu B include:

  • Flu A is much more common (typically 75% of total influenza cases) than flu B.
  • Both flu A and flu B can cause outbreaks and epidemics, but only flu A causes pandemics.
  • Flu A is more severe in adults.
  • Flu B is more common in children, and although it's generally mild to moderate in healthy children, it can be more severe in children under age 5 (possibly due to less previous exposure and therefore lower immunity).
  • Flu A tends to appear earlier in flu season (January and February), while flu B generally occurs later (February and March).
  • Flu A can move from animals, including birds, to people, while flu B develops only in humans.
  • Flu A mutates more quickly than flu B, making it harder to create vaccines that remain effective.


Flu A and flu B have similar symptoms. The severity varies, ranging from mild in many people to severe in some of those who are in high-risk groups, including pregnant people, adults over age 64, children under age 5 (especially infants), and people with underlying health conditions.

Influenza in infants is more likely to be flu A, while flu B is common in school-age children. In most healthy children, both flu A and B have mild to moderate symptoms. But in children under age 5, the symptoms of flu B can be more serious. In adults, flu A can be more severe.

Flu symptoms for both A and B tend to appear in progression, beginning with a sudden onset of fever.

Other early symptoms include:

  • Dry cough
  • Runny nose
  • Sore throat
  • Body aches
  • Headache
  • Fatigue

When young children acquire influenza, they may experience nausea, vomiting, or diarrhea in addition to respiratory symptoms.

Flu symptoms generally last from a few days to about a week or two. But in some people in high-risk groups, the flu can be serious enough to require hospitalization. It can be fatal, usually among people in high-risk groups.

Deaths in the United States from influenza from 2010 to 2020 ranged from 12,000 to 52,000 a year. Hospitalizations associated with influenza ranged from 140,000 to more than 700,000 per year over the same period.


The flu is caused by different strains of the influenza virus. Type A flu viruses are present in animals (such as birds and pigs) in addition to humans, while type B flu viruses only circulate in humans.

Flu A viruses mutate (change genetically) more frequently than flu B. New subtypes of flu A can emerge through mutations and cross to humans from animals. This can result in pandemics (worldwide epidemics) of flu A, which are not seen with flu B.

Flu viruses generally cause more illness during flu season, which is in the fall and winter in each hemisphere. The subtypes that are circulating change from year to year. Flu A is usually more prominent earlier in the flu season, while flu B emerges later in the flu season.

Both flu A and B are usually transmitted by a person with the virus who coughs, sneezes, or transfers their respiratory secretions to surfaces.

Droplets containing the virus are then breathed in by others. Or, the virus may be acquired when a person touches a contaminated surface and then touches their mouth, nose, or eyes.

Both flu A and B are very contagious, and people are most likely to transmit them to others during the first three to four days after feeling ill.


Your healthcare provider may make a diagnosis based on your symptoms. If you are at high risk or having severe symptoms, they may choose to test for the flu and confirm that is the cause. Some types of tests can also identify whether you have type A or B.

The information as to the type or strain of flu is important from a public health perspective but is unlikely to affect your treatment plan. Your provider may be able to do the test on-site, or they may send it to a laboratory. Common types of flu tests include:

  • Rapid influenza diagnostic test (RIDT): This is the most common type of flu test. The sample is collected by nasal swab and tested for influenza viral antigens (substances on the virus that can produce an immune response). Results take about 15 minutes, but the tests may not be as accurate as others.
  • Rapid molecular assay: The sample is taken with a nasal swab and analyzed for its genetic profile. Results take about half an hour.
  • Reverse-transcription polymerase chain reaction (RT-PCR): This is the most accurate type of flu test, which uses a different process to analyze the genetic makeup of a nasal swab sample. It can take up to a few hours to have results. Some of these assays can identify whether the virus is B, A, or a specific subtype of A.

Over-the-counter (OTC) home test kits are available that test for flu A and flu B, respiratory syncytial virus (RSV), and COVID-19. You collect a sample by nasal swab and send it to a laboratory, which will notify you of the results, usually in several days.


Flu A and flu B are treated in the same way. Many people recover from the flu on their own. If your symptoms are bothersome, check with your healthcare provider about the following:

  • Take OTC cough or fever medicine as directed to relieve symptoms (these should not be given to children under age 6 unless directed by their healthcare provider, and aspirin should be avoided in any person under age 19).
  • Get plenty of rest.
  • Stay hydrated.

Home remedies that may help include honey for a sore throat and inhaling steam to clear your stuffy nose. Check with a healthcare provider before you try home remedies.

If your symptoms seem serious, you are pregnant, or you are at high risk for complications of the flu like pneumonia, call your healthcare provider. They may prescribe an antiviral medication that can reduce symptoms and shorten the duration of the flu by about a day.

Flu A and flu B are similar in their response to antiviral medications. Antiviral flu medications work best if you take them within 48 hours of the onset of symptoms. If you need further care or hospitalization, follow your healthcare provider's recommendations.


The Centers for Disease Control and Prevention (CDC) recommends that people 6 months or older receive the flu vaccine each year. Children age 6 months to 8 years may need two injections.

The vaccine protects against both flu A and flu B. The quadrivalent flu vaccine protects against the two subtypes of flu B and two of the subtypes of flu A that are predicted to be circulating.

Because flu A mutates quickly and different strains may be circulating, it is important to get the flu vaccine each year. Also, the vaccine's effectiveness wanes by the end of the yearly flu season.

Other precautions that can help prevent catching or transmitting the flu include:

  • Covering your nose and mouth with a tissue when you cough or sneeze
  • Washing your hands thoroughly and frequently
  • Avoiding close contact with people who may have the flu
  • Staying home if you think you have the flu, so you don't spread it to others


Flu A and B are different types of the influenza virus, and affect adults and children differently. Flu A is more common overall and can be more serious in adults. Flu B is more common in children and may be more serious in young children.

Both types are very contagious and most easily transmitted during the first few days after becoming ill. The symptoms are similar, though they can vary in intensity. You may develop a fever quickly and feel achy and tired. Respiratory symptoms develop later.

Flus A or B can affect certain populations more seriously, so call a healthcare provider if you are pregnant, over age 64, have a weakened immune system, or have a chronic medical illness, such as asthma, heart disease, or diabetes.

A healthcare provider does not need to distinguish between flu A and flu B as treatment is the same. They may diagnose based only on symptoms. In some cases, they may perform a test to determine whether the flu is the cause.

If you think you have the flu, try to stay home, rest, and keep hydrated. Most people recover on their own. Antiviral drugs may help those with severe flu or those who are at high risk. Your best preventive measure is to get a flu vaccine each year, which can help protect against both flu A and B.

16 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Nancy LeBrun
In addition to her extensive health and wellness writing, Nancy has written about many general interest topics for publications as diverse as Newsweek, Teen Vogue, abcnews.com, and Craftsmanship Quarterly. She has authored a book about documentary filmmaking, a screenplay about a lost civil rights hero, and ghostwritten several memoirs.