Antigenic and Genetic Variation of Influenza A(H1N1) Viruses

In 1977, type A(H1N1) influenza virus, related to strains that had circulated in 1950, reappeared and spread worldwide as a true pandemic virus. Summary reports to the World Health Organization showed that after detection in northern China in May 1977, the virus appeared in contiguous countries to the north and south in November of that year and spread across the temperate zones of the Northern Hemisphere during the following winter. Simultaneously, the virus appeared in tropical zones and spread early in the Southern Hemisphere winter season to temperate countries in the south Pacific and south Atlantic zones (Fig. 1).