H2N2

H2N2 is a subtype of the species influenzavirus A (sometimes called bird flu virus). H2N2 has mutated into various strains including the Asian Flu strain (now extinct in the wild), H3N2, and various strains found in birds. It is also suspected of causing a human pandemic in 1889.

Some believe that the 1889 - 1890 Russian flu was caused by the influenzavirus A virus subtype H2N2, but the evidence is not conclusive. It is the earliest flu pandemic for which detailed records are available. "The 1889 pandemic, known as the Russian Flu, began in Russia and spread rapidly throughout Europe. It reached North America in December 1889 and spread to Latin America and Asia in February 1890. About 1 million people died in this pandemic."

The "Asian Flu" was a category 2 flu pandemic outbreak of avian influenza that originated in China in early 1956 lasting until 1958. It originated from mutation in wild ducks combining with a pre-existing human strain. The virus was first identified in Guizhou. It spread to Singapore in February 1957, reached Hong Kong by April, and US by June. Death toll in the US was approximately 69,800. Estimates of worldwide deaths caused by this pandemic varies widely depending on source; ranging from 1 million to 4 million, with WHO settling on "about 2 million".

Asian Flu was of the H2N2 subtype (a notation that refers to the configuration of the hemagglutinin and neuraminidase proteins in the virus) of type A influenza, and an influenza vaccine was developed in 1957 to contain its outbreak.The Asian Flu strain later evolved via antigenic shift into H3N2 which caused a milder pandemic from 1968 to 1969.

Both the H2N2 and H3N2 pandemic strains contained avian influenza virus RNA segments. "While the pandemic human influenza viruses of 1957 (H2N2) and 1968 (H3N2) clearly arose through reassortment between human and avian viruses, the influenza virus causing the 'Spanish flu' in 1918 appears to be entirely derived from an avian source (Belshe 2005)."