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Recent Flu Pandemics

Pandemic Flu in Recent History

During the 20th century, the emergence of several new influenza A virus subtypes caused three pandemics, all of which spread around the world within a year of being detected. The pandemics included:
  • The 1918-1919 "Spanish flu" [A (H1N1)] caused the highest number of known influenza deaths. However, the actual influenza virus subtype was not detected in the 1918-1919 pandemic. More than 500,000 people died in the United States, and up to 50 million people may have died worldwide. Many people died within the first few days after infection, and others died of secondary complications. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in 1977 (see Spanish Flu).
  • The 1957-1958 "Asian flu" [A (H2N2)] caused approximately 70,000 deaths in the United States. The Asian flu was first identified in China in late February 1957, and spread to the United States by June 1957.
  • The 1968-1969 "Hong Kong flu" [A (H3N2)] caused approximately 34,000 deaths in the United States. This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. Influenza A (H3N2) viruses still circulate today.
Viruses that contained a combination of genes from a human influenza virus and an avian influenza virus caused both the 1957-1958 and 1968-1969 pandemics. The 1918 pandemic flu virus now appears to have been avian in origin.
(Click Avian Flu for more information about avian influenza.)


A vaccine probably would not be available in the early stages of a pandemic. When a new vaccine against an influenza virus is being developed, scientists around the world work together to select the virus strain that will offer the best protection against that virus. Manufacturers then use the selected strain to develop a vaccine. Once a potential pandemic strain of influenza virus is identified, it takes several months before a vaccine is widely available. If a pandemic occurs, the U.S. government will work with many partner groups to make recommendations guiding the early use of available vaccine.
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