Table of contents

Influenza virus infection



Influenza is an acute viral infection of the respiratory tract caused by an influenza virus.
Two different genera of the virus family Orthomyxoviridae,influenza A and B, cause a contagious acute respiratory infection in humans. Influenza virus epidemics typically occur during the cold season in temperate regions, when low humidity and temperature ambient conditions are thought to prolong virus shedding and transmission. In subtropical and tropical regions, influenza seasons are less clearly defined, allowing recurrent infections all over the year.
In the US, the annual incidence of influenza is estimated at 5.1% (95% CI, 3.6-6.6%) in adults and 8.7% (95% CI, 6.6-10.5%) in children.
Disease course
Influenza virus infection may result in pneumonia and acute respiratory failure, frequently in the setting of bacterial co-infection. Extra-pulmonary complications of influenza include viral myocarditis and viral encephalitis.
Prognosis and risk of recurrence
The efficacy of the trivalent inactivated vaccine varies according to virus type and subtype. Pooled estimates suggest an estimated efficacy of 59% in adults 18-65 years of age. In patients who are hospitalized with influenza, in-hospital mortality rates are estimated at 9.4%.


Key sources

The following summarized guidelines for the evaluation and management of influenza virus infection are prepared by our editorial team based on guidelines from the Infectious Diseases Society of America (IDSA 2018,2007) and the Association of Medical Microbiology and Infectious Disease Canada (AMMI 2012). ...
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Screening and diagnosis

Indications for testing, flu season, outpatients and ED patients: as per IDSA 2018 guidelines, patients who present with acute onset of respiratory symptoms (with or without fever) and either exacerbation of chronic medical conditions (such as asthma, COPD, or HF) or known complications of influenza (such as pneumonia).
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  • Indications for testing (flu season, hospitalized patients)

  • Indications for testing (low season, outpatients and ED patients)

  • Indications for testing (low season, hospitalized patients)

Diagnostic investigations

Initial diagnostic testing: as per IDSA 2018 guidelines, use rapid molecular assays such as NAATs over rapid influenza diagnostic tests in outpatients to improve detection of influenza virus infection.

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  • Nasopharyngeal specimens

  • Endotracheal specimens

  • Other specimen sites

  • Viral multiplex RT-PCR

  • Viral culture

  • Serologic testing

  • Resistance testing

Medical management

ICU admission criteria: as per AMMI 2012 guidelines, consider hospitalization and admission to the ICU in patients with moderate or severe disease.

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  • Indications for treatment

  • Choice and duration of antiviral agent

  • Adjunctive corticosteroids

  • Adjunctive IV immunoglobulin

  • Management of bacterial co-infection

  • Management of inadequate treatment response

  • Management of outbreaks

Specific circumstances

Intubated patients: as per AMMI 2012 guidelines, administer oseltamivir to intubated patients with influenza illness through a nasogastric tube.
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  • Pregnant patients

  • Immunosuppressed patients

  • Patients with pandemic influenza

Patient education

General counseling: as per AMMI 2012 guidelines, consider counseling together with arrangements for contacts to have medication on hand as an early treatment strategy.

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  • Clinician education

Preventative measures

Influenza vaccination: as per IDSA 2007 guidelines, offer inactivated influenza vaccine to all patients ≥ 50 years of age, others at risk for influenza complications, household contacts of high-risk persons, and healthcare workers.

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  • Pre-exposure prophylaxis

  • Post-exposure prophylaxis