Clinical Guidance

Respiratory illnesses caused by influenza virus infection are difficult to distinguish from illnesses caused by other respiratory pathogens on the basis of signs and symptoms alone. Sensitivity and predictive value of clinical definitions vary, depending on the prevalence of other respiratory pathogens and the level of influenza activity. Uncomplicated influenza illness typically resolves after three to seven days for the majority of persons, although cough and malaise can persist for more than two weeks. The following information has been adapted from the CDCs website.

Transmission and incubation

The typical incubation period for influenza is one to four days (average: two days). Adults shed influenza virus from the day before symptoms begin through five to ten days after illness onset. However, the amount of virus shed, and presumably infectivity, decreases rapidly by three to five days after onset in an experimental human infection model.

Young children also might shed virus several days before illness onset, and children can be infectious for ten or more days after onset of symptoms.

Severely immunocompromised persons can shed virus for weeks or months.

Signs and symptoms

Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (e.g., fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis).

Among children, otitis media, nausea, and vomiting also are commonly reported with influenza illness.

Complications

Influenza virus infections can cause primary influenza viral pneumonia; exacerbate underlying medical conditions (e.g., pulmonary or cardiac disease); lead to secondary bacterial pneumonia, sinusitis, or otitis media; or contribute to coinfections with other viral or bacterial pathogens.

Young children with influenza virus infection might have initial symptoms mimicking bacterial sepsis with high fevers, and febrile seizures have been reported in 6 percent - 20 percent of children hospitalized with influenza virus infection.

Influenza virus infection also has been uncommonly associated with encephalopathy, transverse myelitis, myositis, myocarditis, pericarditis, and Reye syndrome.

Please visit the CDC's Web site for complete clinical information.