The flu can cause complications, especially in patients who have other conditions (e.g., heart disease, asthma, diabetes, cancer), children younger than 2 years of age, and in people over the age of 65. The most common complication is pneumonia, which can be viral, bacterial, or mixed. Primary influenza viral pneumonia is the most serious and difficult-to-treat pulmonary complication.

Other complications include dehydration (occurs when fluid output is higher than fluid intake), bronchitis, and sinus infections (sinusitis). Young children can experience febrile seizures related to infection with the flu.

Other serious flu complications include the following:

  • Encephalopathy (altered brain function or structure)
  • Myocarditis (inflammation of the middle layer of the heart wall)
  • Myositis (muscle inflammation)
  • Pericarditis (inflammation of the membrane that surrounds the heart [pericardium])
  • Reye's syndrome
  • Transverse myelitis (inflammation of the spinal cord)

Reye's syndrome, which is more common in children under the age of 15, can develop about 6 days after infection with the flu or another virus. It affects brain (encephalopathy) and liver (hepatomegaly) function and structure, and causes nausea and vomiting and changes in mental state (e.g., disorientation, agitation, seizure).

Reye's syndrome may be associated with the use of aspirin and is a serious illness—it may cause death in up to 80 percent of cases. The incidence of Reye's syndrome has declined in the United States Since due to warnings about using aspirin in children who have viral illnesses (e.g., flu, chickenpox).

In the United States, serious complications from the flu have increased in recent years, possibly due to an aging population. Primary influenza viral pneumonia is one of the most serious and difficult-to-treat complications. Following severe flu infection, high-risk patients (e.g., elderly patients, patients who have chronic health conditions) may develop serious, irreversible changes in heart, lung, or kidney function.

Study: Children & Flu Risks

In October 2013, the Centers for Disease Control and Prevention (CDC) published the results an 8-year study—October 2004–September 2012—involving flu-related deaths in children under the age of 18 in the United States. According to the CDC study, flu-related deaths occurred in healthy children as well as those with an underlying health condition. Most of children who died from flu complications had not received a seasonal flu vaccine—supporting the CDC's recommendation that all children over the age of 6 months receive an influenza vaccination every year.

To reduce the risk for severe complications, health care providers recommend that parents and child caregivers take influenza seriously in all children and seek prompt medical care for any child who develops flu symptoms. Over the course of the study, one-third of children who died passed away within 3 days of developing symptoms and more than 33 percent died at home, on the way to the hospital, or in the hospital emergency room. Influenza A viruses accounted for 78 percent of flu-related deaths in children and influenza B viruses accounted for about 20 percent.

In addition, the study showed that secondary bacterial infections were actually more common in otherwise healthy children than in children with underlying health problems. This finding supports the use of antiviral medications in children who develop the flu. Parents and child caregivers should follow their doctors' advice to prevent flu infection in children and reduce flu complications.

Flu Prognosis

In most cases, patients recover completely from the flu within a couple of weeks. Children who are younger than age 5; people over the age of 65; and patients who have heart, lung, or kidney problems, diabetes, or cancer have a higher risk for complications from the flu.

Publication Review By: the Editorial Staff at

Published: 08 Feb 2007

Last Modified: 04 Dec 2014