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Between 1889 and 1977 six pandemics of influenza have
been recognized. Based on
serological studies, the 1989-90 pandemic was thought to be caused by H2N? (the
N component has not been definitely determined), and the 1899-1900 pandemic was
thought to be caused by H3N8. The
serotypes H2N? and H3N8 have never been isolated from humans.
These identifications were made only by
serological studies. The
pandemic of 1918-19 was caused by H1N1; this was also based on studies of sera
obtained from persons who lived through the 1918-19 pandemic period, using a
prototype H1N1 virus isolated from human as antigen. H1N1 was replaced by H2N2; the new serotype emerged to cause
the Asian flu pandemic of 1957. Eleven
years later, another new influenza virus, H3N2 emerged to cause the Hong Kong
flu pandemic of 1968. The H3N2
serotype has persisted up to the present time.
However, H1N1 reappeared (reemergence?) to cause the pandemic of 1977
(Russian flu). Currently both H3N2
and H1N1 circulate together in the population.
It is of note that all three pandemics, 1957, 1968 and 1977 originated in
China.
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