Influenza A tends to cause worse symptoms than influenza B. B viruses also change slower genetically compared to influenza A viruses. Some evidence suggests that some infections from influenza B can be just as severe as influenza A.

What are the stages of flu B?

Influenza B generally occurs in stages, such as: Incubation : This is the period during which the flu virus replicates in your body. You typically won't have symptoms, but it's important to note that you can be contagious. Early symptoms: Symptoms typically develop within two to three days after you come into contact with the virus and come on quickly. You might have a fever, chills, coughing, muscle aches, and sore throat. Peak illness: These symptoms usually peak after two to three days after onset. A fever goes away within three to four days, but it can last up to eight days in some people. Recovery: Most people with mild illness get better within one to two weeks. A lack of energy and coughing can linger for weeks after recovery.

How long will flu B last?

Flu symptoms generally get better within one week. Keep in mind that how long the flu lasts can be shorter or longer in some people.

What Are the Symptoms of Influenza (Flu) B?

By

Leah Groth

Updated on October 18, 2024

Medically reviewed by

Jane Kim, MD

Sick woman sitting on the couch and blowing her nose and drinks hot drink — Oksana Shufrych / Getty Images

The Centers for Disease Control and Prevention (CDC) reported that influenza B accounted for 31% to 35% of confirmed cases in May 2024. Influenza B is a type of flu virus that causes symptoms in humans only. Symptoms of flu B often include a fever, coughing, and headache.

Influenza B is known to cause seasonal epidemics along with influenza A, which is typically responsible for more severe illnesses. Most flu cases—around 70% of reported cases—are influenza A. The influenza B virus is the cause of the next most common type of flu.

Influenza B Symptoms

The symptoms of influenza B are similar to those of other strains of influenza A. Some of the most common influenza B symptoms include:

Coughing
Fever
Headache

Influenza B largely affects the respiratory system. It can also cause symptoms that involve the entire body. You may also develop gastrointestinal symptoms, although they're less common.

Respiratory Symptoms

The flu is a respiratory infection, meaning it targets the lungs, nose, and throat. Some of the most common respiratory symptoms include:

Congestion, or a stuffy nose
Coughing
Runny nose
Sneezing
Sore throat

These symptoms are similar to other respiratory infections, such as the cold, but can be more severe. The flu can lead to complications, such as bronchitis or pneumonia, or worsen symptoms in people with asthma.

Body Symptoms

One of the most common systemic (whole-body) symptoms of the flu is a fever. It typically lasts three to four days and can reach 106 degrees in some people.

Other body symptoms may include:

Chills
Body or muscle aches
Headache
Lack of energy

Gastrointestinal Symptoms

Some evidence suggests that gastrointestinal symptoms are more common with influenza B than influenza A, but the data is mixed. These symptoms can include:

Abdominal pain
Loss of appetite
Nausea
Vomiting

Keep in mind that these symptoms are different from the “stomach flu,” which isn’t actually caused by flu viruses. This infection is medically known as viral gastroenteritis, which is commonly caused by norovirus and rotavirus.

Causes

The flu spreads when a person with the illness emits infectious droplets by coughing, sneezing, or talking. These droplets can land in your mouth or nose if you're standing close to them. You can also pick up infectious droplets that land on an object or surface, but it's less common.

Diagnosis

A healthcare provider can diagnose the flu based on the information you give them about your symptoms and if it's prevalent in your area. They can also use molecular and rapid antigen tests to diagnose the illness.

Molecular tests provide rapid and highly accurate results to see if you have the flu. A healthcare provider may get a nose or mouth fluid sample to send to a lab for testing.

A healthcare provider will use rapid antigen tests to get a flu diagnosis confirmation in less than an hour. These tests are typically less accurate than molecular tests.

Influenza B Treatment

The goal of treatment is to help reduce and resolve flu symptoms. Treatment options include antiviral medications and self-care measures.

Antiviral Medications

Antiviral medications can cut down the time you are sick with the flu. A healthcare provider may prescribe one of the following:

Rapivab (peramivir)
Relenza (zanamivir)
Tamiflu (oseltamivir phosphate)
Xofluza (baloxavir marboxil)

These medications aren't necessary for all flu cases, especially if you have mild symptoms. A healthcare provider may only prescribe them if you become very sick or have a higher risk of flu-related complications.

Self-Care

You can usually treat mild flu symptoms at home with the following self-care measures:

Drinking plenty of liquids
Getting a lot of rest
Taking medications like Advil (ibuprofen) or Tylenol (acetaminophen) to reduce a fever or over-the-counter (OTC) cold medicines
Using cough drops or throat sprays for a sore throat

Prevention

The best way to prevent the flu—both influenza A and B—is by getting the flu vaccine. The recommendation is for people 6 months or older to get vaccinated against the flu. Children aged 8 and younger need two doses during flu season. Those aged 9 and older would only need one dose.

You can also prevent the flu by:

Keeping your distance if you or someone else is sick
Regularly washing your hands with soap and water for at least 20 seconds
Staying home when you have symptoms
Using hand sanitizer with at least 60% alcohol if soap and water are unavailable

Complications

The main complication of the flu is pneumonia. Other possible complications include brain infection, meningitis, and seizures.

Some people have a higher risk of flu-related complications, including those who:

Are aged 2 and younger or 65 years and older
Are more than three months pregnant
Have diabetes, a weak immune system, or chronic heart, kidney, or lung conditions
Live in a long-term care facility

When To Contact a Healthcare Provider

You don't have to see a healthcare provider for mild flu symptoms, regardless of the type of virus causing them. It's important to contact a healthcare provider, in contrast, if you're at risk of complications.

You'll also want to see a healthcare provider if flu symptoms worsen and don't improve with home remedies. Get medical attention right away if you or your child have:

High fever
Shortness of breath
Trouble breathing

A Quick Review

Influenza B is a type of flu caused by viruses from the Victoria or Yamagata lineages. Symptoms are similar to other strains of the flu, but they're mild in comparison. The flu is both treatable and preventable. Talk to a healthcare provider if you have problems with treatment and symptoms or may be at risk for complications.

Frequently Asked Questions

Is flu A or B worse?

Influenza A tends to cause worse symptoms than influenza B. B viruses also change slower genetically compared to influenza A viruses. Some evidence suggests that some infections from influenza B can be just as severe as influenza A.
What are the stages of flu B?
Influenza B generally occurs in stages, such as:
- Incubation: This is the period during which the flu virus replicates in your body. You typically won't have symptoms, but it's important to note that you can be contagious.
- Early symptoms: Symptoms typically develop within two to three days after you come into contact with the virus and come on quickly. You might have a fever, chills, coughing, muscle aches, and sore throat.
- Peak illness: These symptoms usually peak after two to three days after onset. A fever goes away within three to four days, but it can last up to eight days in some people.
- Recovery: Most people with mild illness get better within one to two weeks. A lack of energy and coughing can linger for weeks after recovery.
How long will flu B last?

Flu symptoms generally get better within one week. Keep in mind that how long the flu lasts can be shorter or longer in some people.

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