Influenza virus is notoriously liable to change its antigenic composition. These changes can affect both its main surface antigens, the haemagglutinin (H) and the neuraminidase (N). A subtype with the composition H1N1 was in circulation from 1947 to 1957 when it was replaced by Asian subtype (H2N2). In the last decade the Hong Kong (H3N2) strains have become prevalent. The recent reappearance of an H1N1 strain was first detected in China in May 1977 and in the Soviet Union at the end of 1977. Many people born before 1957 therefore have residual protective antibodies to both antigens, but children and young adults have had no experience of an H1N1 strain and are susceptible. Indeed, nearly all the outbreaks which it caused were in the young, and schools especially were widely affected.1 The virus closely resembles a strain which circulated in 1950 and which is presumed to have remained dormant.2 It is not clear how important the 1977 H1N1 virus will be as an agent of human disease. Many young people were infected in Britain last winter and much immunity must have been acquired, but enough susceptible people are probably left to make a further epidemic possible.
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