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For further information, see CMDT Part 32-06: Common Viral Respiratory Infections

Key Features

Essentials of Diagnosis

  • Cases usually in epidemic pattern

  • Onset with fever, chills, malaise, cough, coryza, and myalgias

  • Aching, fever, and prostration out of proportion to catarrhal symptoms

  • Leukopenia

General Considerations

  • An orthomyxovirus transmitted by respiratory droplets

  • Three antigenic subtypes that infect humans

    • Types A and B produce identical clinical symptoms

    • Type C produces milder disease

  • Pandemics usually due to type A infections with major antigenic shift (large genetic reassortment of the virus)


  • Up to 5 million cases of severe influenza are estimated by the WHO to occur annually, with up to 0.5 million annual deaths

  • Annual epidemics and rare pandemics appear at varying intervals, usually in the fall or winter affecting 10–20% of the global population on average each year

  • Incidence highest in school-age children and young adults, students, prisoners, day care and health care workers; persons with asthma are at particular risk

  • Complications occur most often in elderly, immunocompromised individuals

Clinical Findings

Symptoms and Signs

  • Abrupt onset

  • Systemic symptoms

    • Fever (lasts 3–5 days [range, 1–7 days])

    • Chills

    • Headache

    • Malaise

    • Myalgias

  • Respiratory symptoms

    • Rhinorrhea

    • Congestion

    • Pharyngitis

    • Hoarseness

    • Nonproductive cough

    • Substernal soreness

  • Gastrointestinal symptoms and signs may occur, particularly among young children with influenza B virus infections

  • Elderly patients may present with lassitude and confusion, often without fever or respiratory symptoms

  • Signs include mild pharyngeal injection, flushed face, and conjunctival redness

  • Moderate enlargement of the cervical lymph nodes and tracheal tenderness may be observed

Differential Diagnosis

  • Common cold

  • Primary bacterial pneumonia

  • Respiratory syncytial virus

  • Mycoplasma infection

  • Pertussis

  • Legionnaires disease

  • Parainfluenza infections

  • Adenovirus

  • Enterovirus

  • Coronavirus, including SARS-CoV-2

  • Flavivirus

  • Acute HIV infection

  • Cytomegalovirus

  • Epstein-Barr virus

  • Atypical dengue


Laboratory Tests

  • Rapid influenza diagnostic tests (RIDT)

    • Widely available for detecting influenza antigens from nasal or throat swabs

    • Highly specific

    • Produce fast results but have low sensitivity leading to high false-negative results

    • Not all commercial RIDT can differentiate between influenza A and influenza B

    • None of the available RIDT can provide information on influenza A subtypes

  • Digital immunoassays and rapid nucleic acid amplification tests

    • More sensitive than traditional rapid influenza diagnostic tests

    • However, the sensitivity of newer PCR techniques is compromised early in the season during low prevalence periods

    • A nasopharyngeal swab, nasal aspirate, combined nasopharyngeal swab with oropharyngeal swab, or material from a bronchoalveolar lavage can be tested for any influenza strain

  • When influenza pneumonia is suspected, lower respiratory tract specimens should be collected and tested for influenza viruses by reverse transcription polymerase chain reaction (RT-PCR) or digital immunoassays or rapid diagnostic tests




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