1957 and 1968 Pandemic

The viruses causing the 1957 ("Asian Flu", H2N2) and 1968 ("Hong Kong Flu", H3N2) pandemics are both believe to have originated in Asia. There is good genetic evidence that both these viruses were re-assortments: viruses that had been generated from mixing of human and influenza viruses.

These pandemics had much lower mortality than the 1918 pandemic, causing approximately 2 and 1 million deaths, respectively. (Note that these estimates are based on limited data, and that various other estimates have been reported)

It is possible that the later pandemics were less severe than the 1918 pandemic because the viruses were partly of human origin. The 1968 pandemic may have been milder than the 1957 one because the NA type stayed the same and there may have been some immunity to the NA component. However, it is usually the HA that is believed to be the main immune target and in the vaccines it is only the HA content that is assessed for potency of vaccines.