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In the United States, April 2009, a brand new Flu strain occurs in California. This flu strain was never before detected in humans or animals. Upon scientific investigation, the virus gene that made up H1N1 was most related to North American and Eurasian Swine origin.

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Centers for Disease Control and Prevention

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In the spring of 2009, a novel influenza A (H1N1) virus emerged. It was detected first in the United States and spread quickly across the United States and the world. This new H1N1 virus contained a unique combination of influenza genes not previously identified in animals or people. This virus was designated as influenza A (H1N1)pdm09 virus. Ten years later work continues to better understand influenza, prevent disease, and prepare for the next pandemic.

The (H1N1)pdm09 virus was very different from H1N1 viruses that were circulating at the time of the pandemic. Few young people had any existing immunity (as detected by antibody response) to the (H1N1)pdm09 virus, but nearly one-third of people over 60 years old had antibodies against this virus, likely from exposure to an older H1N1 virus earlier in their lives. Since the (H1N1)pdm09 virus was very different from circulating H1N1 viruses, vaccination with seasonal flu vaccines offered little cross-protection against (H1N1)pdm09 virus infection. While a monovalent (H1N1)pdm09 vaccine was produced, it was not available in large quantities until late November-after the peak of illness during the second wave had come and gone in the United States. From April 12, 2009 to April 10, 2010, CDC estimated there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (range: 195,086-402,719), and 12,469 deaths (range: 8868-18,306) in the United States due to the (H1N1)pdm09 virus.

Since the 2009 H1N1 pandemic, the (H1N1)pdm09 flu virus has circulated seasonally in the U.S. causing significant illnesses, hospitalizations, and deaths.

Additionally, CDC estimated that 151,700-575,400 people worldwide died from (H1N1)pdm09 virus infection during the first year the virus circulated.** Globally, 80 percent of (H1N1)pdm09 virus-related deaths were estimated to have occurred in people younger than 65 years of age. This differs greatly from typical seasonal influenza epidemics, during which about 70 percent to 90 percent of deaths are estimated to occur in people 65 years and older.

Though the 2009 flu pandemic primarily affected children and young and middle-aged adults, the impact of the (H1N1)pdm09 virus on the global population during the first year was less severe than that of previous pandemics. Estimates of pandemic influenza mortality ranged from 0.03 percent of the world's population during the 1968 H3N2 pandemic to 1 percent to 3 percent of the world's population during the 1918 H1N1 pandemic. It is estimated that 0.001 percent to 0.007 percent of the world's population died of respiratory complications associated with (H1N1)pdm09 virus infection during the first 12 months the virus circulated.

The United States mounted a complex, multi-faceted and long-term response to the pandemic, summarized in The 2009 H1N1 Pandemic: Summary Highlights, April 2009-April 2010. On August 10, 2010, WHO declared an end to the global 2009 H1N1 influenza pandemic. However, (H1N1)pdm09 virus continues to circulate as a seasonal flu virus, and cause illness, hospitalization, and deaths worldwide every year.

Historical Resources

Read a summary of key events of the 2009 H1N1 pandemic and CDC's response activities between April 2009-April 2010.

Learn more about the timeline of major events that took place during the 2009 H1N1 pandemic.

View images taken during CDC's response to the 2009 H1N1 pandemic.

Videos from 2009 H1N1 Pandemic

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A summary of key events of the 2009 H1N1 pandemic and the CDC's response activities between April 2009 and April 2010.

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Two children of ages 8 and 10, located in California, presented as the first documented cases. Total cases approximately 60.8 million by April, 2010. Due to the H1N1 pandemic, United States faced 12, 469 deaths. By April 18th, 2009, Infections were reported to the Center for Disease Control and World Health Organization. By April 21st, CDC reports to the U.S. of new strain and work toward developing a vaccine. The diagram below isolates regions of the United States illustrating the number of cases developed during the pandemic.

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All of the United States and territories reported largest amount of cases worldwide.

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Children, teenagers, middle age adults that were 65 years and younger. This graph shows the number of deaths impacted by H1N1 in the United States related to age group.

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From April, 2009 to April 2010

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Centers for Disease Control and Prevention

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A timeline of major events that took place during the 2009 H1N1 flu pandemic.

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Upper respiratory infection, fever, coughing, sore throat, chills, headaches, muscle pain, fatigue, diarrhea, vomiting, pneumonia.

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Antiviral medications including Tamiflu, Relenza as well as IV Peramivir for emergency cases. Hospitalization for 5 days is recommended for severe cases or high risk patients such as children, pregnant women. By October 5th, 2009, the first vaccination was given to prevent the spread of the virus.