What To Know About Influenza B (Flu)

Influenza B is one of the two main types of influenza (flu) viruses that cause seasonal outbreaks of flu. Similar to influenza A, type B flu is highly contagious and can cause a range of symptoms, such as fever, cough, headache, and chills.

Estimates suggest that 3% to 11% of the U.S. population develops either influenza A or influenza B each year. Influenza B is the less common type and accounts for about 25% of all flu cases. Both type A and type B flu can cause severe symptoms, though most people recover within a week without treatment. If your condition isn't improving, treatments can help you feel better more quickly.

Types of Influenza  

Researchers have identified four classes of influenza viruses, including:

• Influenza A: Flu A virus is the most common type of flu and causes about 75% of all flu infections. There are two main subtypes of influenza A, based on specific proteins: hemagglutinin (H) and neuraminidase (N). Flu A is the only type that causes both epidemics (seasonal outbreaks) and pandemics (rapid and global spread of a new disease).
• Influenza B: Flu B virus accounts for about 25% of all flu cases and is more common in children over the age of five. This type has two lineages—B/Yamagata and B/Victoria. This type of flu spreads more slowly than influenza A.
• Influenza C: Type C virus can affect people, but only causes mild illness.
• Influenza D: Type D virus doesn’t affect people or cause any symptoms. This type of influenza primarily causes symptoms in cattle, but can also spread to other animals.
  

Flu B Symptoms

The symptoms of influenza B are similar to those of influenza A—and both types of flu can cause a range of mild to severe illness. While influenza B is less common, it can cause more serious symptoms among young children.

Symptoms of influenza B can occur anywhere from one to four days after exposure to the virus. Typical signs of influenza B include:

• Body aches and muscle pain
• Congestion
  
• Headache
• Fatigue
• Nausea and vomiting
• Runny or stuffy nose
• Sore throat
• Sudden onset of fever or chills

In most cases, symptoms will subside within a week. But sometimes, a sore throat and cough can linger for a couple of weeks.

What Causes Influenza B?

You may develop influenza B when the type B virus infects your respiratory system—the nasal passages, mouth, throat, and lungs. When your body becomes exposed to a virus, your immune system works hard to kill the virus and stop the germs from spreading. Most of the symptoms of flu develop as a result of your immune response to infection.

In most cases, you'll likely develop influenza B by inhaling germs and droplets released into the air when an infected person coughs, sneezes, or talks near you. You can also pick up this virus by touching infected surfaces or making contact with an infected person.

Risk Factors

Anybody can develop influenza B, but certain factors can also increase your risk of illness. For example, having certain health conditions can compromise your immune system and make it difficult for your body to fight off the flu quickly. These conditions include:

• Asthma
• Chronic obstructive pulmonary disease (COPD)
• Cancer
• Diabetes
• Heart disease
• HIV/AIDS
• Pregnancy

Additionally, your age can also play a role in your likelihood of developing the flu.

Diagnosis

Based on symptoms alone, healthcare providers can’t confirm an influenza B diagnosis. In addition to taking your medical history, there are several diagnostic tests that your provider may order to confirm your diagnosis:

• Rapid influenza diagnostic test (RIDT): RIDTs detect genetic fragments that cause an immune response (known as antigens) from influenza A and B. They can detect these fragments by collecting a sample of your DNA from a swab of your throat or nose. Usually, the results of this test are ready within 15 minutes.
• Rapid molecular assay: Another test that produces results in 15 minutes is the rapid molecular assay. This exam detects the genetic material of influenza to see if your body has been infected by the virus.
• Molecular assays: Other molecular assays, such as reverse transcription-polymerase chain reaction (RT-PCR) tests can also detect the presence of influenza B in your system. You can get the results of these types of tests between 45 minutes and a few hours.
• Immunofluorescence: This diagnostic exam uses a specialized, fluorescent microscope to detect signs of influenza. Your healthcare provider will take a sample of your mucus or DNA from your nasopharynx (the top part of your throat behind the nasal passages) for the test. This test can take several hours for you to get your results back.

Treatments for Flu B

In most cases, your immune system can fight off influenza infection with at-home remedies, like rest and staying hydrated. However, if your symptoms are severe, your healthcare provider may recommend more aggressive treatments.

At-Home Remedies

Getting enough rest and staying hydrated is the first line of treatment for the flu. It can also help to stay home from work, school, or social commitments and only go out for medical necessity. If you go out while sick, experts recommend wearing a mask to prevent spreading the infection to others.

Over-the-Counter (OTC) Medications

If at-home remedies alone don't help improve your symptoms, taking over-the-counter medicines can help treat symptoms like headache, cough, and fever. Common OTC medications include Tylenol (acetaminophen) or non-steroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen). If you're thinking about OTC medicines for your child, it's best to contact their pediatrician for recommendations.

Antiviral Drugs

There are several antiviral medications, which act directly on the influenza virus. These drugs can reduce the severity of your symptoms and decrease the length of your illness:

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

Prevention

Every year, flu vaccines are developed to tackle the latest strains of the virus. Getting vaccinated helps prevent you from developing the virus and can reduce the duration and severity of your illness if you do experience symptoms of the flu. There are several types of vaccines, but for the 2045-2025 flu season, all are trivalent—meaning they protect against one flu B and two flu A strains.

You can also lower your risk of getting the flu by:

• Avoiding touching your face, nose, eyes, or mouth
• Disinfecting and cleaning surfaces
• Limiting contact with people who are sick
• Thoroughly washing your hands with soap and water
  

Complications

If influenza B causes severe symptoms and you do not seek treatment, your condition may lead to some complications. Potential complications of flu B include:

• Ear infections
  
• Encephalitis, or inflammation of the tissues in your brain
• Muscle inflammation
• Organ damage or failure, particularly in your lungs and kidneys
• Pneumonia
• Sepsis, which can be a life-threatening emergency
• Sinus infections
  

A Quick Review

Influenza B is a common type of flu virus that causes symptoms like fever, headache, chills, cough, congestion, and body aches. This type of flu is highly contagious. While anyone can develop flu type B, young children and people with underlying health conditions have the highest risk. Fortunately, influenza B usually goes away on its own within a week—with proper rest and hydration. However, if your symptoms are ongoing, your healthcare provider may prescribe medications to treat the illness.