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Clinical manifestations and diagnosis of pandemic H1N1 influenza ('swine influenza')

Anna R Thorner, MD
Section Editor
Martin S Hirsch, MD
Deputy Editor
Elinor L Baron, MD, DTMH


In late March and early April 2009, an outbreak of H1N1 influenza A virus infection was detected in Mexico, with subsequent cases observed in many other countries, including the United States [1,2]. In June 2009, the World Health Organization (WHO) raised its pandemic alert level to the highest level, phase 6, indicating widespread community transmission on at least two continents [3]. The pandemic was declared to be over in August 2010 [4].

The clinical manifestations and diagnosis of pandemic H1N1 influenza A virus infection will be reviewed here. The epidemiology, treatment, and prevention of pandemic H1N1 influenza A virus infection are discussed separately. (See "Epidemiology of pandemic H1N1 influenza ('swine influenza')" and "Treatment and prevention of pandemic H1N1 influenza ('swine influenza')".)

The clinical manifestations and diagnosis of seasonal and avian (H5N1) influenza viruses are discussed separately. (See "Clinical manifestations of seasonal influenza in adults" and "Seasonal influenza in children: Clinical features and diagnosis" and "Clinical manifestations and diagnosis of avian influenza" and "Diagnosis of seasonal influenza in adults".)


The following case definitions have been provided by the United States Centers for Disease Control and Prevention (CDC) [5]:

Influenza-like illness (ILI) is defined as fever (temperature of 100ºF [37.8ºC] or greater) with cough or sore throat in the absence of a known cause other than influenza.


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Literature review current through: Feb 2015. | This topic last updated: Mar 20, 2015.
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