Seroepidemiology of H1N1 influenza: the range of the affected population is widening.

The H1N1 influenza epidemic during the winter of 1980-81 affected a wider range of the population than the epidemics of 1977-78 and 1978-79. The increasing frequency of infection among elderly people can be explained by the antigenic drift in the surface proteins of the virus. Small children and pregnant women were affected to only a small extent by the first and second outbreaks, but they suffered more in 1980-81. This altered epidemiology may be explained by adaptation of the virus to the human host, but the genetic mechanism remains an open question.