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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 1899-90 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. Recently,
the 1918 virus was further identified as H1N1 by complete sequencing of
hemagglutinin and neuraminidase from formalin-fixed, paraffin-embedded, lung
tissue samples from patients died of influenza during the 1918 pandemic, as well
as from a frozen sample obtained by in situ biopsy of the lung of a victim
buried in permafrost in Alaska since 1918. Phylogenetic analysis suggested that
the H1N1 virus is intermediate between mammals and birds and might have been
adapted in humans before the 1918 pandemic. 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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