Influenza, Influenza-Associated Mortality, and Novel Influenza A

Clinical Description

Influenza is an acute viral disease of the respiratory tract characterized by abrupt onset of fever, myalgia, headache, severe malaise, nonproductive cough, sore throat and rhinitis. Without laboratory confirmation, influenza is referred to as influenza-like illness (ILI).

Clinical Case Definition

Without laboratory confirmation, the ILI case definition used by CDC for national surveillance is fever > 100° Fahrenheit or 37.8° Celsius and cough and/or sore throat (in the absence of a known cause). Influenza is commonly recognized by epidemiologic characteristics. Influenza illness may be indistinguishable from other viral respiratory illnesses based on symptoms alone.

Laboratory Criteria for Diagnosis

• Virus isolation by cell culture
• CLIA certified laboratory Immunofluorescence, Influenza Enzyme Immuno Assay (EIA), or reverse transcription-polymerase chain reaction (RT-PCR)
• CLIA waived Commercial rapid non-culture diagnostic tests for influenza virus
• *Four-fold rise in antibody titer between the acute and convalescent serum specimens.
   

Case Classification

Confirmed: Laboratory confirmation by one of the test methods listed above.

*Note: Serologic testing for influenza is available in a limited number of laboratories, and should
only be considered as evidence of recent infection if a four-fold rise in influenza (HI) antibody
titer is demonstrated in paired sera. Single acute serum samples are not interpretable and are not
considered to be confirmatory.