History of influenza pandemics
Global pandemics have been observed for several hundred years. The best documented pandemics occurred in 1918 (A(H1N1), Spanish flu), 1957 (A(H2N2), Asian flu) and 1968 (A(H3N2), Hong Kong flu). All three pandemics varied in severity.
Spanish influenza is estimated to have infected 50% of the world's population, half of which suffered a clinical infection. The total excess mortality was estimated at 40-50 million, giving a mortality rate of 2-2.5%. The infection and mortality rates were highest among healthy adults (20 to 40-years-old).
Asian influenza affected some 40-50% of the world’s populations during two waves of the pandemic. Of the affected people, 25-30% experienced clinical disease. The mortality rate was estimated at approximately 1 in 4 000 and the total death toll likely exceeded 1 million.
The morbidity and mortality of Hong Kong influenza was as high as that seen in the 1957-1958 Asian influenza pandemic, showing a range of 1-3 million fatalities. Both Asian and Hong Kong influenza arose in southeast Asia.