Influenza, or the flu, is a highly infectious respiratory illness. As well as the usual symptoms, there are a number of possible complications that may occur with the flu, some of which can be quite dangerous.

Each flu season, the flu affects around 8% of the population of the United States.

Certain groups of people have a higher risk of developing moderate-to-severe complications from the flu. These groups include young children, older adults, pregnant people, and individuals with chronic underlying medical conditions.

This article outlines some possible complications of the flu, alongside their associated treatment options. It also provides information on the people most at risk of developing flu complications and offers advice on when to contact a doctor.

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Image credit: Luis Alvarez/Getty Images

Complications from the flu can range from moderate to severe. In some cases, they can be life threatening.

The sections below will look at some of these potential flu complications in more detail.

Sinus and ear infections

The flu virus may cause inflammation and infection of the sinuses or ears.

Symptoms of a sinus infection include:

  • mucus in the throat
  • nasal congestion
  • a runny or drippy nose
  • throat irritation
  • green or yellow nasal mucus
  • headaches at the front of the head
  • tenderness in the face, particularly around the nose area
  • tooth or upper jaw pain
  • bad breath
  • fever
  • fatigue

Symptoms of an ear infection include:

  • earache
  • itching or irritation in the ear
  • a feeling of fullness or pressure in the ear
  • discharge from the ear
  • difficulty hearing
  • fever


Treatment options for a sinus or ear infection may include:


Bronchitis is the medical term for inflammation of the large airways in the lungs. The condition can be short term (acute) or long term and recurrent (chronic). The flu virus usually causes acute bronchitis.

Symptoms of bronchitis include:


Most cases of acute bronchitis clear up within a few weeks without the need for medical treatment.

However, the following home treatments may help aid recovery or alleviate symptoms:


The flu can cause various symptoms that may lead to fluid loss and dehydration.

Some examples of these symptoms include:

Symptoms of dehydration include:

  • dark yellow urine
  • less frequent urination
  • dry skin, mouth, or eyes
  • a lack of tears in infants


People who have the flu should drink plenty of fluids to help prevent dehydration.

Also, the following options will help replace lost electrolytes:

  • broth
  • fruit juices
  • sports drinks

People with severe dehydration will require intravenous (IV) fluids in a hospital.


Pneumonia is the medical term for an infection in one or both lungs.

The flu virus can invade the lungs, causing viral pneumonia. It can also cause a secondary, or superimposed, bacterial infection that results in bacterial pneumonia.

Symptoms of pneumonia include:

  • a cough that produces green, yellow, or bloody mucus
  • shortness of breath or rapid, shallow breathing
  • chest pain
  • an inability to take a deep breath
  • fever
  • nausea and vomiting
  • loss of appetite
  • fatigue
  • confusion


Treatment options for pneumonia depend on the type of pneumonia a person has and its severity.

Some possible treatment options include:

  • antibiotics to treat bacterial pneumonia
  • pain relievers, such as acetaminophen and ibuprofen
  • rest
  • fluids

People who develop severe pneumonia are likely to require treatment in a hospital. For example, they may require IV fluids, oxygen, or breathing assistance using a mechanical ventilator.

Inflammation of the heart, brain, or muscles

In rare cases, the flu virus can cause infection and inflammation of the brain, heart, or muscles. An infection of the brain or heart can result in severe and potentially life threatening complications.

Encephalitis is the medical term for inflammation of the brain. Symptoms may include:

  • headache
  • confusion
  • vision problems
  • fever
  • seizures

Myocarditis is the medical term for inflammation of the heart. Symptoms may include:

Myositis is the medical term for inflammation of the muscles. Symptoms may include:

  • muscle weakness
  • muscle tenderness
  • muscle swelling
  • difficulty carrying out everyday tasks, such as:
    • climbing stairs
    • rising from a chair
    • lifting or holding objects


Some potential treatment options for encephalitis and myocarditis include:

  • antiviral medications to target viral causes, such as herpes
  • antibiotics to target bacterial causes
  • steroid injections to suppress the immune system and control inflammation
  • immunoglobulin therapy, which uses drugs to help control the immune system

Treatment options for myositis include:


Sepsis is an extreme and potentially life threatening immune system response to infection. In sepsis, the immune system goes into overdrive and begins attacking healthy cells, tissues, and organs in the body.

The flu virus can cause sepsis or a secondary bacterial infection that leads to sepsis.

Some possible signs and symptoms of sepsis include:

  • an unusually high or low temperature
  • low blood pressure
  • an increased heart rate
  • shortness of breath
  • severe pain or discomfort
  • mental changes, such as confusion or unusual sleepiness


Without prompt treatment, sepsis can lead to organ failure and death.

A person who has sepsis will require immediate hospitalization. They should receive antibiotic treatment as soon as a healthcare professional makes a diagnosis.

Some people may also require breathing assistance using a ventilator or surgery to remove areas of infection.

Worsening of existing health conditions

The flu can also worsen certain chronic health conditions, including:

Some conditions can also weaken the immune system, making it harder for a person’s body to fight the flu virus. Examples of these conditions include diabetes and HIV.

Having an existing medical condition also increases the risk of developing flu complications.


Managing any existing conditions and receiving antiviral drugs as soon as flu symptoms develop can reduce symptoms and help prevent complications.

Certain factors — such as age, medication use, and underlying health issues — can affect how the body responds to the flu virus.

According to the Centers for Disease Control and Prevention (CDC), people in the following groups are at increased risk of developing flu complications:

  • people over the age of 65 years
  • children under the age of 5 years, particularly those under the age of 2 years
  • people under the age of 19 years who take long-term medications containing aspirin or salicylate
  • people taking corticosteroids or medications to suppress the immune system
  • pregnant people

The CDC also notes that the following health conditions can also increase a person’s risk of developing flu complications:

People at risk of developing flu complications should ensure that they receive annual vaccines.

The CDC recommends the annual flu vaccine for everyone over 6 months of age and anyone who is at risk of developing complications from the flu.

The CDC also recommends the pneumonia, or pneumococcal, vaccine for children under 2 years of age, adults over 65 years of age, people with certain medical conditions, and people who smoke cigarettes.

If a person is at risk of developing flu complications, they should contact a doctor as soon as they experience symptoms. Receiving antiviral drugs within 2 days of developing symptoms can reduce symptom severity and duration and lower the risk of complications.

The CDC recommends that children receive immediate medical attention if they experience any of the following symptoms:

  • a change in breathing or difficulty breathing
  • a blue tinge to the face or lips
  • muscle pain, which may make them not want to walk
  • recurrent cough or fever
  • a lack of alertness
  • seizures
  • a fever higher than 104°F (40°C) in children over 12 weeks of age or a fever of any kind in children under 12 weeks of age
  • any symptoms of dehydration, such as:
    • a lack of urine for 8 hours
    • a dry mouth
    • crying without tears
  • worsening of existing health conditions

The CDC recommends that adults seek immediate medical attention for any of the following symptoms:

  • difficulty breathing or shortness of breath
  • persistent pain in the chest or abdomen
  • recurrent cough or fever
  • muscle pain
  • weakness or unsteadiness
  • dehydration
  • dizziness
  • a new state of confusion or an inability to wake
  • seizures
  • worsening of existing health conditions

The flu is a highly contagious respiratory illness that may lead to complications. Certain groups of people are at increased risk of developing flu complications. These include young children, older adults, and people with underlying health conditions.

People should contact a doctor straight away if they are in an at-risk group and they develop any symptoms of the flu.

Receiving the annual flu vaccine reduces a person’s risk of getting the flu. For people who do get the flu, receiving antiviral medications as soon as symptoms appear may help reduce symptom severity and duration as well as lower the risk of severe complications.