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  • Acute, typically self-limited, febrile infection caused by orthomyxovirus influenza types A and B
  • Marked by inflammation of nasal mucosa, pharynx, conjunctiva, and respiratory tract
  • Outbreaks have varying degrees of severity and generally peak in winter.
  • Influenza virus can undergo antigenic shift (abrupt change) leading to viral strains with little immunologic resistance in a population. This can result in pandemic outbreaks. Minor seasonal variations are called antigenic drift.
  • System(s) affected: typical cases: head/eyes/ears/nose/throat, pulmonary; complicated cases: cardiac and CNS involvement
  • Synonym(s): flu; grippe; acute catarrhal fever


  • Predominant age: children (3 months to 16 years) and young adults
    • Morbidity: seasonal morbidity and rates of hospitalization highest in very young (preschool), elderly (>75 years of age), and individuals with comorbid illness (lung disease, malignancy)
  • Predominant sex: male = female

  • Seasonal influenza in preuniversal vaccination: 95 million cases per year, typically fall/winter
  • Attack rates in healthy children: 10–40% each year, prior to routine influenza vaccination
  • Weekly reports:

Etiology and Pathophysiology

Orthomyxovirus (influenza types A [majority] and B); influenza A virus subtypes HxNx based on hemagglutinin and neuraminidase

  • Incubation is 1 to 4 days; infected persons are most contagious during peak symptoms.
  • Spread by aerosolized droplets or contact with respiratory secretions
  • Hemagglutinin binds to columnar respiratory epithelium where replication occurs, and neuraminidase protein facilitates spread along respiratory epithelium.

Risk Factors

  • For contracting disease
    • Crowded environments such as nursing homes, barracks, schools, and correctional facilities
  • For complications
    • Neonates, infants, elderly
    • Pregnancy, especially in 3rd trimester
    • Chronic pulmonary diseases
    • Cardiovascular diseases, including valvular pathology and congestive heart failure (CHF)
    • Metabolic disease, morbid obesity
    • Hemoglobinopathies
    • Malignancy; immunosuppression
    • Neuromuscular diseases that limit respiratory function and ability to handle secretions

General Prevention

  • Vaccination: All persons >6 months should be vaccinated annually unless contraindication present.
    • Inactivated influenza vaccine (IIV) is available with either 3 (IIV3) or 4 (IIV4) strains of influenza. IIV also is available as high-dose, intradermal, cell culture–based (ccIV3), MF59-adjuvanted (aIIV3), and recombinant hemagglutinin vaccine (RIV3).
    • Live attenuated influenza vaccine (LAIV) is an intranasal quadrivalent vaccine.
  • IIV recommended annually for the following:
    • All persons aged ≥6 months
    • Vaccine should be administered annually as soon as the vaccine is available.
    • Protection occurs 1 to 2 weeks after immunization.
    • Typically, mild side effects include low-grade fever and local reaction at vaccination site.
    • Inactivated IM dose: ≥3 years of age: 0.5 mL; children 6 to 35 months of age: 0.25 mL
    • Intradermal formulation for 18- to 64-year-olds uses a short 30-gauge needle in a single-use prefilled syringe with 0.1 mL vaccine; somewhat higher local reactions when given intradermal
    • Single annual dose except for children <9 years of age, who should receive 2 doses (4 weeks apart) the 1st year they receive influenza vaccine
    • Vaccine contraindication: Severe allergy such as anaphylaxis to IIV components, allergies from eggs are not considered a contraindication; observe all patients for 15 minutes after vaccination; no skin testing with influenza vaccine is needed in egg-allergic patients. RIV is safe in patients with an egg allergy.
    • Precaution: Guillain-Barré syndrome within 6 weeks after a previous dose of influenza vaccine
  • LAIV is recommended for 2018 to 2019 influenza season.
  • IIV-HD: high-dose quadrivalent IIV
    • Contains 4 times the antigen concentration of IIV
    • Licensed for persons ≥65 years of age
    • Results in higher antibody levels but somewhat higher rates of local reactions
    • Advisory Committee on Immunization Practices does not express a preference for/against IIV-HD.
  • Antiviral prophylaxis depends on current resistance patterns each year; see for patterns or check with local health department.
    • In high-risk groups that have not been vaccinated or need additional control measures during epidemics; not a substitute for vaccination unless vaccine is contraindicated (1)[A]
    • During influenza season, for those with contraindications to vaccine who have been exposed to the virus (1)[A]
    • For staff and residents in nursing home outbreaks
    • For immune-deficient persons who are expected not to respond to vaccination after viral exposure
Pediatric Considerations
  • Vaccinate children 6 months and older annually.
  • Recommend all household members with children <6 months be vaccinated (2)[A].
  • For children who need 2 doses, administer first dose as soon as available for second dose to be given before the end of October (2)[A].
  • For prophylaxis, oseltamivir dosage varies by weight and is recommended by the CDC for prophylaxis for children ≥3 months; zanamivir is approved for prophylaxis for children ≥5 years of age at a dosage of 2 inhalations per day. Prophylaxis treatment duration is 7 days. For prophylaxis, the dosage of amantadine and of rimantadine is 5 mg/kg/day up to 150 mg in 2 divided doses. Currently, amantadine and rimantadine are not recommended due to resistance.
Pregnancy Considerations
  • The CDC recommends vaccinating all women who will be pregnant during influenza season.
  • If unvaccinated at the time of flu season, pregnant women should receive IIV or RIV.
  • Oseltamivir, zanamivir, peramivir, rimantadine, and amantadine are pregnancy Category C.

Commonly Associated Conditions

Bacterial pneumonia

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