In order to understand the spectrum of severity of the disease caused by swine influenza A(H1N1) virus infection , the clinical case description includes both mild form of influenza-like llness (ILI) and more severe forms (lower respiratory tract infections including pneumonia and severe acute respiratory illness (SARI)). In addition, asymptomatic laboratory-confirmed infections should be reported.
The following case definitions are for the purpose of reporting probable and confirmed cases of wine influenza A(H1N1) virus infection to WHO.
Clinical case description
Acute febrile respiratory illness (fever >38°C ) with the spectrum of disease from influenza-like llness to pneumonia.
1. A Confirmed case of swine influenza A(H1N1) virus infection is defined as an individual with laboratory confirmed swine influenza A(H1N1) virus infection by one or more of the following tests*:
• real-time RT-PCR
• viral culture
• four-fold rise in swine influenza A(H1N1) virus specific neutralizing antibodies.
2. A Probable case of swine influenza A(H1N1) virus infection is defined as an individual with an influenza test that is positive for influenza A, but is unsubtypable by reagents used to detect seasonal influenza virus infection OR
An individual with a clinically compatible illness or who died of an unexplained acute respiratory illness who is considered to be epidemiologically linked to a probable or confirmed case.
* Note: The test(s) should be performed according to the most currently available guidance on testing (http://www.who.int/csr/disease/swineflu/en/index.html).
The complete document of the WHO case definition is available here