• The incubation period of influenza virus is approximately 2 to 3 days.
• Invasion of ciliated columnar epithelial cells by the influenza virus leads to necrosis of the ciliated epithelial lining of the upper and lower respiratory tracts, as well as a subsequent inflammatory response.
• Pneumonia is a result of direct invasion of the organism as well as secondary bacterial infection.

• Although influenza affects persons of all ages, the highest morbidity and mortality occurs in infants and the elderly.
• Epidemics of influenza occur almost exclusively during winter months, peak approximately 2 weeks after the index case, and last 4 to 8 weeks. Up to 75% of schoolchildren in the epidemic region may be affected.
• Transmission of influenza virus occurs by aerosol droplets as well as by direct or indirect contact.

• Secondary bacterial infections (10% of children): bacterial pneumonia (pneumococcal or staphylococcal), otitis media, sinusitis
• Primary progressive viral pneumonia : pulmonary hemorrhage, high morbidity and mortality rates
• Acute myositis during convalescent period is most commonly associated with influenza B infection: rhabdomyolysis, myoglobinuria, elevated transaminase levels
• Reye syndrome : fatty degeneration of the liver and diffuse encephalopathy; more commonly associated with influenza B infection, although can occur after influenza A infection as well; association between aspirin use during acute illness
• Febrile convulsions
• Drug toxicity: Influenza infection may result in increased serum levels of certain medications that are metabolized by the liver (i.e., theophylline).
• Rare sequelae in severe cases of influenza infection include focal and diffuse myocarditis, diffuse cerebral edema, mediastinal lymph node necrosis, sudden death, and encephalitis.

• Pharyngitis
• Laryngotracheitis (croup)
• Bronchitis
• Bronchiolitis
• Pneumonia
• Gastroenteritis
• Conjunctivitis

• Viral infections, including but not limited to respiratory syncitial virus (RSV), parainfluenza, adenovirus
• Streptococcus pyogenes infection
• Bacterial sepsis in young infants

• Infection with the influenza virus causes distinct clinical pictures based on the age of the affected individual.
• Infants and young children may suffer higher fevers and more severe respiratory symptoms.
• Many older children and adults infected with influenza are diagnosed with a “viral respiratory infection,” without specific reference to the viral etiologic agent.
• The diagnosis of influenza infection is more commonly made in light of previously identified index cases or specific findings such as myositis.

• Abrupt onset of illness, beginning with dry cough, coryza
• Fever, headache, anorexia, malaise, myalgias, sore throat, irritability
• Respiratory complaints range from mild cough to severe respiratory distress (infants).
• Gastrointestinal complaints in younger children may include vomiting, diarrhea, and severe abdominal pain.

• Cough is the predominant respiratory sign. Infants and small children may exhibit a “barky” cough (croup).
• Nasal congestion and conjunctival and pharyngeal infections are common.
• Cervical adenopathy is more common in children than in adults.
• Neonates may appear septic: apnea, circulatory collapse, petechiae.
• A generalized macular or maculopapular rash is sometimes observed.
• The myositis that accompanies the convalescent phase of influenza infection is commonly limited to or most severe in the gastrocnemius and soleus muscles. These patients may present with inability to walk or toe-walking.

• Patients considered to be at high risk for severe disease include those with asthma or other chronic pulmonary disease, hemodynamically significant cardiac disease, immunosuppressed children, and persons traveling to areas where an influenza outbreak is presently occurring.
• Patients considered at possible high risk for severe manifestations of influenza include those with HIV infection, sickle cell anemia, diabetes mellitus, chronic renal disease, or chronic metabolic disease.
• Patients considered likely to dangerously transmit influenza infection include hospital personnel, especially if there is contact with children or any high-risk patients; household contacts of high-risk patients, including those with HIV; and persons residing in dormitories or other institutional settings.
• Infection with the influenza virus may trigger exacerbations of asthma.

• Viral culture from nasopharyngeal secretions will be positive within 2 to 6 days.
• Rapid immunofluorescent tests utilizing monoclonal antibodies have variable sensitivity (70%–100%), but high specificity (100%). These are more reliable for influenza A. Polymerase chain reaction (PCR) assays are also available.
• Serologic evidence of infection involves comparison of acute and convalescent serum antibody titers (6 months). ELISA testing is now available for influenza.

• The chest radiographs in patients with lower airway involvement are indistinguishable from other viral lower respiratory infections.
• Chest radiographs may be normal despite significant respiratory involvement.

• The leukocyte count in patients with influenza may be high, low, or normal.
• The differential count is too variable to be of help in diagnosis.
• Evaluation of arterial oxygenation by arterial blood gas analysis or, preferably, pulse oximetry may be required in severe cases of influenza infection. Occasional infants without roentgenographic evidence of lower respiratory tract infection have experienced apnea or rapid decrements in pulmonary function.

• Diagnosis of severe viral disease
• Discovery of index cases of epidemics

• Most patients with influenza infection require supportive oral hydration, antipyresis, and routine decongestant therapy.
• Antitussive medications should be used cautiously, and should be appropriate to the age of the child.

• Humidified air, with oxygen as needed, will be helpful to most patients with respiratory symptoms of influenza.
• Supplementary airway maneuvers, including endotracheal intubation, may be required for severe laryngotracheitis or patients with hypoxia that is unresponsive to high-flow oxygen administration.
• Hypovolemic and distributive causes of poor peripheral circulation respond well to intravascular volume repletion.

• Chemotherapy against influenza is recommended for patients with severe disease or children at high risk of severe illness or complications (see Special Questions).
• Amantadine hydrochloride (<9 yrs or <40 kg: 5 mg/kg/day in 1–2 divided doses; >40 kg: 200 mg/day in 1–2 divided doses) has in vitro activity against influenza A. The few pediatric studies available show some efficacy in reducing the severity of symptoms if amantidine is administered within 48 hours of symptom onset. Rimantidine, a synthetic analog of amantidine, is approved for prophylactic use only (see Prevention). Neither medication is approved for use in infants less than 1 year of age or for influenza B infection.
• Ribavirin has been used successfully as an aerosol medication in the treatment of both influenza A and influenza B; however, it is not currently approved for treatment of influenza in children.

• Amantidine dose should be reduced in patients with renal insufficiency. The side effects of amantidine include insomnia, lightheadedness, and difficulty concentrating.
• Patients with epilepsy have a higher risk of seizure activity when receiving amantidine.

• It is necessary to provide annual vaccination to high-risk individuals and persons likely to transmit influenza infection to high-risk individuals. These groups are delineated in the list of patients under Special Questions.
• Children who are receiving chronic aspirin therapy should be considered for vaccination because of the associations between aspirin use, influenza infection, and Reye syndrome.

• Prophylactic administration of amantidine is recommended for certain subgroups of patients:
  • High-risk children who are exposed to influenza A infection less than 2 weeks after influenza vaccination was given (see Special Questions)
  • Immunocompromised patients (poor response to vaccine)
  • High-risk patients who cannot receive the vaccine (anaphylactic reaction to chicken or eggs)
  • Control of outbreaks in institutions housing high-risk persons
• The dose of amantidine for prophylaxis is the same as the treatment dose. For children over 20 kg, 100 mg/d is also acceptable.

• Fever associated with influenza infection usually lasts up to 5 days. Recrudescence of fever does not necessarily signify the onset of a secondary bacterial infection.
• Cough may last up to 2 weeks.
• Lethargy or malaise may persist for up to 2 weeks.
• Influenza A infection usually lasts longer than influenza B or influenza C infections.

• Clinical signs of secondary bacterial infection (see Complications)
• Deteriorating mental status or respiratory status after initial improvement
• Myoglobinuria in the face of muscle pain

Q: When is it safe for a child with influenza to return to daycare or school?
A: Older children with influenza may shed the virus in nasal secretions for up to 7 days from onset of symptoms, and younger children even longer. Therefore, older children with influenza may return to school 1 week after the onset of symptoms, and infants and toddlers should remain home for 10 to 14 days.

Q: Can a child on chronic steroid therapy be immunized against influenza?
A: In general, children who require maintenance steroid therapy for their underlying illness should still receive influenza immunization. If possible, immunize while the child is on the lowest possible dose of steroids and not during a period of high-dose therapy.

Q: What are the chances of acquiring influenza despite annual vaccination?
A: Vaccination against influenza is greater than 70% effective in preventing disease and greater than 90% effective in preventing death from the infection.

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Feiste JE, Mitchell JM, Sullivan DB. After the flu: acute viral myositis. Contemp Pediatr 1995;(12):29–51.

Gruber WC. Influenza viruses. In Long SS, Pickering LK, Prober CG, eds. Principles and practice of pediatric infectious diseases. New York: Churchill Livingstone, 1997, 1267–1274.

Piedra PA. Influenza virus pneumonia: pathogenesis, treatment, and prevention. Semin Respir Infect 1995;10:216–223.

Prevention and control of influenza: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1991;41(RR-9):1–17.