H1N1 Flu

Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
The English language content on this website is being archived for historic and reference purposes only.
For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

NOTE:This page is not intended as a stand-alone Web document and is intended to serve merely as a Section 508-accessible version of the PowerPoint presentation "2009 H1N1: Overview of a Pandemic, April 2009 - August 2010."

2009 H1N1: Overview of a Pandemic

Detection of the 2009 H1N1 Virus

March 2009
- 2 cases of febrile respiratory illness in children (un-related, no pig contact)
- Residents of adjacent counties in southern California, ill in late March
2009 pandemic influenza A (H1N1) virus testing began at CDC on April 15th
Both viruses genetically identical
- Contain a unique combination of gene segments previously not recognized among swine or human influenza viruses in the United States

Image of Morbidity and Mortality Weekly Report Headline that reads 'Swine Influenza A (H1N1) Infection in Children – Southern California, March-April 2009'. On April 21, this report was posted as an MMWR Early Release on the MMWR website

Retrospective evidence of respiratory illness outbreaks in Mexico (February/March)

April 26, 2009
U.S. declares National Public Health Emergency

June 11, 2009
WHO declares Global pandemic of novel influenza A (H1N1) virus

Novel Swine Influenza Detected

1) April 15, 2009
- 10-year-old boy
- Mild flu symptoms
- Part of CDC study using “Mesoscale” flu test
2) April 17, 2009
- 8-year-old girl
- Mild flu symptoms
- Part of CDC Border Influenza Project
Surveillance showed no other novel flu cases
No suspect flu cases in hospitals in the region

Image of a United States map with a red box drawn around the area of southern California. Red lines drawn from the red box to another larger red box indicate the area is being shown in greater detail in the second box. Inside the second box, there are two red dots numbered 1 and 2, indicating the two locations in southern California where novel swine influenza was initially detected.

Novel Swine Influenza Detected

Both cases reported in MMWR - MMWR 58(15);400-02
Posted gene sequences on GenBank
Searched for swine exposure
- 10 year old visited San Diego zoo
- 8 year old at county fair
No evidence to link to swine

Image of contestant sitting alongside a pig at a county fair in California. A red dot over the picture indicates CDC investigated the situation.

Contestant at County Fair in California

June 11, 2009

"On the basis of available evidence, and these expert assessments of the evidence, the scientific criteria for an influenza pandemic have been met. I have therefore decided to raise the level of influenza pandemic alert from phase 5 to phase 6. The world is now at the start of the 2009 influenza pandemic."

- Dr Margaret Chan, Director-General of the World Health Organization

Image of line drawn to represent the passage of time, with blocks of time depicted in different colors

Surveillance Systems

As the outbreak investigation expanded, CDC epidemiologists worked with state, local and international partners to enhance CDC’s existing flu surveillance systems to better track the spread of 2009 H1N1