An acute febrile illness characterized by fever, malaise, and respiratory symptoms


  • Although influenza affects people of all ages, the highest morbidity and mortality occur in young children <2 years old, the geriatric population, and those with high-risk conditions.
  • Influenza epidemics occur almost exclusively during winter months, peak ~2 weeks after the index case, and last 4 to 8 weeks.
  • Attack rates are highest among school-aged children (range 10–40%).
  • An estimated 10–20% outpatient visits among children <5 years old attributable to influenza
  • Transmission of influenza virus occurs via large respiratory droplets or contact with contaminated surfaces.
  • After an incubation period of 1 to 4 days, viral shedding starts 24 hours before symptom onset and usually continues for 7 days.
    • Prolonged shedding in young children and immunocompromised individuals

Risk Factors

High-risk conditions for severe disease include the following:

  • Chronic pulmonary disease (i.e., asthma)
  • Hemodynamically significant cardiac disease
  • HIV and other immunodeficiencies
  • Chronic immunosuppressive therapy
  • Hemoglobinopathies (i.e., sickle cell disease)
  • Long-term salicylate use
  • Chronic renal dysfunction
  • Chronic metabolic disease, morbid obesity
  • Neuromuscular disorders

General Prevention

  • Vaccination
    • Routine influenza vaccination for ALL individuals ≥6 months old
  • Prioritize vaccination for those at highest risk for influenza complications and their close contacts, including (see Risk Factors):
    • Young children ages 6 to 59 months
    • Older adults ≥50 years, adults, and children with certain chronic diseases and high-risk conditions
    • Long-term care facility residents
    • American Indians/Alaska Natives
    • Pregnant women
    • Health care professionals
    • Out-of-home caregivers
    • Household contacts of all children <5 years old OR children 5 to 18 years old with high-risk conditions
  • Vaccine types
    • Trivalent inactivated influenza vaccine (TIV) Afluria® (Seqirus) for ages ≥9 years and Fluvirin® (Seqirus) for ages ≥4 years
    • Quadrivalent influenza vaccines newly available for 2013 to 2014 season and beyond; include second influenza B strain
      • Inactivated: Fluarix® (GlaxoSmithKline) for ages ≥3 years, Fluzone® (Sanofi Pasteur), and Flulaval® (Seqirus) for ages ≥6 months
      • Live-attenuated influenza vaccine (LAIV) FluMist® (MedImmune), Fluzone® (Sanofi Pasteur); for healthy nonpregnant 2- to 49-year-olds; administered as an intranasal spray
      • Cell culture–based inactivated vaccine Flucelvax® (Seqirus) for ages ≥4 years
    • Recombinant trivalent hemagglutinin vaccine Flublok® (Protein Sciences) for ages ≥18 years; egg-free
    • High-dose (Fluzone® High-Dose, Sanofi Pasteur) and adjuvanted (Fluad®, Seqirus) trivalent-inactivated vaccines available for older adults ≥65 years
  • Special vaccination considerations:
    • There is no preferential recommendation for one vaccine product over another for children who are eligible for more than one available vaccine.
    • Children ≤8 years old receiving seasonal influenza vaccination for the first time should receive 2 doses of vaccine at least 4 weeks apart.
  • LAIV not recommended for individuals with high-risk conditions, young children (ages 2 to 4 years) with history of wheezing in past year, contacts of severely immunocompromised persons (such as contacts of BMT patients in a protected environment), or children receiving chronic aspirin therapy
  • Contraindications to vaccination: history of severe allergic reaction to any vaccine component or after previous influenza vaccine dose
    • Precautions: Those with history of Guillain-Barré syndrome within 6 weeks of previous influenza vaccination should consult a physician before receiving the vaccine.
  • Influenza vaccines can be safely administered to children with history of egg allergy (see for specific recommendations).
  • Postexposure chemoprophylaxis
    • Indicated for high-risk children who are unvaccinated or were vaccinated within 2 weeks of exposure, immunocompromised patients who have a poor vaccine response, or to control outbreaks in institutions housing high-risk people
    • Chemoprophylaxis should begin within 48 hours of exposure to be most effective.


  • Orthomyxoviruses influenza types A, B, and C. Influenza C virus has not been reported as a cause of influenza epidemics.
  • Influenza A subtypes defined by two surface antigens: hemagglutinin and neuraminidase
    • Currently circulating subtypes include pH1N1 (pmd09) and H3N2.
  • Mild variation, or antigenic drift, for both A and B viruses results in seasonal epidemics; antigenic shift occurs only with A viruses and results in pandemics.

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