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Influenza virus infection

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Updated 2024 WHO guidelines for the diagnosis and management of influenza virus infection .

Background

Overview

Definition
Influenza is an acute viral infection of the respiratory tract caused by an influenza virus.
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Pathophysiology
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.
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Epidemiology
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.
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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.
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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%.
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Guidelines

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 American College of Obstetricians and Gynecologists (ACOG 2024), the Center for Disease Control (CDC 2024), the World Health Organization (WHO 2024), the American Academy of Allergy, Asthma & Immunology (AAAAI/ACAAI 2018), and the Infectious Diseases Society ...
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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).
B
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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 WHO 2024 guidelines:
Consider obtaining point-of-care influenza NAAT or influenza digital immunoassay in patients with suspected non-severe influenza virus infection at high risk of progression to severe influenza.
C
Consider obtaining high sensitivity and high specificity tests (NAAT or PCR) in patients with suspected severe 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

Indications for treatment
As per WHO 2024 guidelines:
Initiate treatment with antiviral agents in all patients tested positive for influenza virus infection.
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Initiate treatment if high sensitivity and specificity tests are available but results will be delayed for > 24 hours, and discontinue treatment if the test is negative.
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  • Choice and duration of antiviral agent

  • Adjunctive corticosteroids

  • Other therapies

  • Management of bacterial coinfection

  • Management of inadequate treatment response

  • Management of outbreaks

Specific circumstances

Pandemic influenza: as per IDSA 2007 guidelines, test for H5N1 infection in patients who have an illness compatible with influenza and known exposure to poultry in areas with previous H5N1 infection.
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  • Pregnant patients

Preventative measures

Influenza vaccination, indications: as per CDC 2024 guidelines, offer routine annual influenza vaccination with an age-appropriate influenza vaccine in all persons aged ≥ 6 months without any contraindications.
E
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  • Influenza vaccination (patients with egg allergy)

  • Pre-exposure prophylaxis

  • Post-exposure prophylaxis