What Is Influenza B Symptoms? Risks And Treatment

Influenza B is one of two types of influenza viruses that cause the seasonal flu in humans. It infects the nose, throat, and lungs, and while it is often described as milder than influenza A, it can still lead to serious illness, hospitalization, and death. The symptoms are similar to other flu types: sudden fever, body aches, fatigue, cough, and sore throat. The main risks come from complications like pneumonia, particularly in young children, older adults, and people with chronic health conditions. Treatment focuses on managing symptoms and, in some cases, using antiviral medications within the first two days of illness.

What Is Influenza B and How Does It Differ From Influenza A?

Influenza viruses are divided into four types: A, B, C, and D. Types A and B are the ones that cause seasonal epidemics in people. Influenza C causes mild illness, and influenza D mostly affects cattle.

The key difference between A and B is host range and mutation rate. Influenza A infects many animals, including birds, pigs, and horses. This allows it to undergo major genetic shifts, which is how pandemics start. Influenza B primarily infects humans and mutates more slowly.

Influenza B is split into two main lineages: Victoria and Yamagata. Both can circulate during flu season, and the seasonal flu vaccine is designed to protect against both lineages along with two strains of influenza A.

Clinically, the symptoms of A and B are nearly identical. Some research suggests influenza B may cause more muscle pain or fatigue in certain age groups, but the difference is not significant enough to guide diagnosis. You cannot tell which type you have based on symptoms alone. A laboratory test is required.

What Are the Most Common Symptoms of Influenza B?

Flu symptoms typically come on suddenly, unlike a cold which develops gradually. Most people infected with influenza B will experience several of the following:

• Fever or chills (not everyone gets a fever)
• Cough, often dry and persistent
• Sore throat
• Runny or stuffy nose
• Muscle and body aches
• Headache
• Fatigue, which can be severe
• Vomiting and diarrhea (more common in children than adults)

Symptoms usually appear one to four days after exposure to the virus. Most healthy people recover in about one to two weeks without medical treatment. The cough and fatigue can linger for several weeks even after other symptoms resolve.

Fever in adults is typically 100°F (37.8°C) or higher. In children, fevers can run higher. If fever lasts more than a few days or resolves and then returns, that can signal a secondary bacterial infection and warrants medical evaluation.

Who Is at Highest Risk for Severe Illness From Influenza B?

Influenza B can make anyone sick, but certain groups face a higher risk of complications. The Centers for Disease Control and Prevention (CDC) identifies these groups as high risk:

• Children younger than 5 years, especially those under 2
• Adults 65 years and older
• Pregnant women and women up to two weeks postpartum
• Residents of nursing homes and long-term care facilities
• People with chronic conditions such as asthma, diabetes, heart disease, or weakened immune systems
• People with a body mass index (BMI) of 40 or higher

Children under 5 are particularly vulnerable to influenza B. Studies have shown that influenza B causes a disproportionate number of pediatric flu deaths compared to influenza A, especially in healthy children. No one knows exactly why, but it may be related to the fact that children have less prior immunity to B strains.

Adults over 65 tend to have weaker immune responses to vaccination and infection. Chronic inflammation and underlying organ dysfunction also make it harder for older bodies to fight off the virus.

What Are the Complications of Influenza B?

The most common serious complication is pneumonia. It can be viral pneumonia caused directly by the influenza virus or bacterial pneumonia that develops as a secondary infection. Bacterial pneumonia often appears after the initial flu symptoms start to improve, with a sudden return of fever, cough, and chest pain.

Other complications include:

• Myocarditis (inflammation of the heart muscle)
• Encephalitis (inflammation of the brain)
• Sepsis (a life-threatening response to infection)
• Worsening of existing conditions like asthma, COPD, or congestive heart failure
• Ear infections and sinus infections, especially in children

Reye’s syndrome is a rare but serious complication that affects children and teenagers recovering from a viral illness. It is strongly associated with giving aspirin to children with flu or chickenpox. Never give aspirin to anyone under 19 with a viral illness unless a doctor specifically directs it.

How Is Influenza B Treated?

For most healthy people, treatment is supportive care. Rest, fluids, and over-the-counter medications to reduce fever and aches are the mainstays. Acetaminophen and ibuprofen are commonly used. Do not use aspirin in children or teenagers due to the risk of Reye’s syndrome.

Antiviral drugs are available by prescription. The most commonly used are oseltamivir (Tamiflu), zanamivir (Relenza), and baloxavir marboxil (Xofluza). These drugs work best when started within 48 hours of symptom onset. They can shorten illness duration by about one day and may reduce the risk of complications, though the evidence for this benefit is stronger in high-risk patients.

Antivirals are recommended for people at high risk of complications, people with severe illness, and people who are hospitalized. Some clinicians also prescribe them for healthy people who want to shorten their illness, but this is a judgment call between patient and doctor.

Antibiotics do not work against influenza B because it is a virus. They are only prescribed if a bacterial complication like pneumonia develops.

When Should You See a Doctor?

Most people with flu do not need to see a doctor. But certain warning signs require prompt medical attention.

In adults, seek care if you experience:

• Difficulty breathing or shortness of breath
• Chest pain or pressure
• Confusion or sudden dizziness
• Severe weakness or inability to stand
• Dehydration (not urinating, dry mouth, dizziness when standing)
• Fever that improves then returns with a worse cough

In children, seek emergency care if you notice:

• Fast breathing or trouble breathing
• Bluish lips or face
• Not drinking enough fluids
• Not waking up or not interacting
• Fever with a rash
• Irritability so severe the child does not want to be held
• Flu symptoms that improve then return with fever and worse cough

In infants younger than 3 months, a fever of 100.4°F (38°C) or higher is always a reason to call a doctor immediately.

How Can You Prevent Influenza B?

The seasonal flu vaccine is the best protection. It is updated each year to match circulating strains. The vaccine does not guarantee you will not get the flu, but it significantly reduces the risk of severe illness, hospitalization, and death. It also reduces the risk of spreading the virus to others.

The CDC recommends annual flu vaccination for everyone 6 months and older, with rare exceptions. The vaccine is especially important for people in high-risk groups and for those who live with or care for high-risk people.

Beyond vaccination, everyday preventive actions matter. Wash your hands frequently with soap and water or use alcohol-based hand sanitizer. Avoid touching your eyes, nose, and mouth. Avoid close contact with sick people. If you are sick, stay home until at least 24 hours after your fever is gone without the use of fever-reducing medication.

Wearing a mask in crowded indoor settings during flu season can also reduce transmission, especially if you are at high risk or caring for someone who is.

How Long Does Influenza B Last and When Are You Contagious?

Most people are contagious from about one day before symptoms start until five to seven days after becoming sick. Young children and people with weakened immune systems can shed the virus for longer and may be contagious for more than a week.

The illness itself typically lasts one to two weeks. Fever usually resolves within three to five days. The cough and fatigue can persist for two to three weeks. If symptoms last longer than two weeks or worsen at any point, a doctor should evaluate you for complications.

Is Influenza B Worse Than Influenza A?

There is no simple answer. Both can cause severe illness and death. In some seasons, influenza B is associated with more pediatric deaths relative to the number of infections. In other seasons, influenza A dominates hospitalizations.

What matters more than the type is your individual risk profile and how quickly you seek care. A healthy adult with influenza B will likely recover at home. An older adult with heart disease and influenza B may need hospitalization. The virus type matters less than the host.

Can You Get Influenza B More Than Once in a Season?

Yes. Because there are two lineages of influenza B (Victoria and Yamagata), it is possible to get infected with one lineage, recover, and then get infected with the other lineage in the same season. It is also possible to get influenza A and influenza B in the same season. The immunity you develop to one strain does not fully protect you against the other.