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Home Criteria Infectious Disease Case Definition for Influenza A (H1N1) Virus Infection (Swine Flu)


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Case Definition for Influenza A (H1N1) Virus Infection (Swine Flu) Print E-mail
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Written by G. Firman MD   
Monday, 13 July 2009 07:08


After identification of novel influenza A (H1N1) virus infection in Mexico, a case definition was developed. The initial definition of suspected novel influenza A (H1N1) virus infection included any hospitalized patient with severe acute respiratory illness. On May 1, 2009, this definition was expanded to include any person with acute respiratory illness defined as fever and either sore throat or cough. On May 11, 2009, the definition of suspected case was changed again to include any person with:

  1. fever,
  2. cough, and
  3. headache,

plus at least one of the following:

  1. rhinorrhea,
  2. coryza,
  3. arthralgia,
  4. myalgia,
  5. prostration,
  6. sore throat,
  7. chest pain,
  8. abdominal pain, or
  9. nasal congestion.

In children aged <5 years, irritability replaced headache.

A laboratory-confirmed case of novel influenza A (H1N1) virus infection was defined as illness in any person who had a respiratory specimen that tested positive for novel influenza A (H1N1) by real-time reverse transcriptase PCR (rRT-PCR).



  1. Centers for Disease Control and Prevention (CDC). Update: novel influenza A (H1N1) virus infection - Mexico, March-May, 2009. MMWR Morb Mortal Wkly Rep. 2009 Jun 5;58(21):585-9. [Medline]



Created: Jul 13, 2009

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