As of August 2010, H1N1 flu is no longer a pandemic. This fact sheet provides historical information about H1N1 flu. It will no longer be updated.
H1N1 flu is a respiratory infection. It can cause mild to severe illness.
The H1N1 flu is caused by a virus. It spreads by:
Pandemic H1N1 Virus
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The H1N1 flu is more common in people under 25 years old. The main risk for H1N1 flu is contact with a person who has it.
The risk for severe H1N1 infection is higher with:
Symptoms of H1N1 flu may range from mild to severe. They may be:
If H1N1 flu becomes severe, it can cause pneumonia.
The doctor will ask about your symptoms and health history. Diagnosis may be based on symptoms.
A nose or throat swab may be taken and tested. This can confirm the diagnosis.
The goal is to ease symptoms and prevent problems. Those with severe illness may need hospital care.
Treatment options may be:
The risk of H1N1 flu may be reduced by:
Centers for Disease Control and Prevention
World Health Organization
Public Health Agency of Canada
2009 H1N1 flu. Centers for Disease Control and Prevention website. Available at: https://www.cdc.gov/h1n1flu. Accessed April 7, 2021.
Influenza in adults. EBSCO DynaMed website. Available at: https://www.dynamed.com/condition/influenza-in-adults. Accessed April 7, 2021.
Lee RU, Phillips CJ, et al. Seasonal influenza vaccine impact on pandemic H1N1 vaccine efficacy. Clin Infect Dis. 2019;68(11):1839-1846.
Seasonal influenza: flu basics. Centers for Disease Control and Prevention website. Available at: https://www.cdc.gov/flu/about/index.html. Accessed April 7, 2021.
Traveler's health. Centers for Disease Control and Prevention. Available at: https://wwwnc.cdc.gov/travel. Accessed April 7, 2021.
Last reviewed December 2020 by David L. Horn, MD, FACP
Last Updated: 4/7/2021