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. Few young people had any existing immunity (as detected by antibody response) to the (H1N1)pdm09 virus, but nearly one-third of people over the age of 60 years had antibodies against this virus, likely from an exposure to an older H1N1 virus earlier in their lives. The (H1N1)pdm09 virus was very different from H1N1 viruses that were circulating at that time; vaccination with seasonal flu vaccines thus 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, which was 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 that there were 60.8 million cases (range: 43.3-89.3 million), 274,304 hospitalizations (195,086-402,719), and 12,469 deaths (8868-18,306) in the United States due to the (H1N1)pdm09 virus.* CDC estimated that between 151,700 and 575,400 people worldwide died from 2009 H1N1 virus infection during the first year the virus circulated.** Globally, CDC estimated that 80 percent of (H1N1)pdm09 virus-associated deaths were in people younger than 65 years of age, which differs from typical seasonal influenza epidemics during which about 70 percent to 90 percent of deaths are estimated to occur in people 65 years of age and older.
Though this most recent influenza pandemic primarily affected children and young and middle-aged adults, the impact of (H1N1)pdm09 virus on the global population overall 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 the (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 influenza virus and cause illness and deaths worldwide every year.
2009 Additional Resources
Influenza virology and animal transmission studies
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