Seroepidemiology of H1N1 influenza: the infection and re-infection rate in winter 1978-79.

It was observed that small children and pregnant women were affected to only a small extent by the H1N1 influenza outbreak of winter 1978-79. This supports earlier findings from the epidemic season of 1977-78 and demonstrates that the evolutionary changes in the epidemic virus were not reflected in any appreciable way in this curious phenomenon. The frequency of elderly subjects possessing antibodies against the epidemic H1N1 virus was low, and virtually equal in the pre-epidemic and post-epidemic sampling. This low attack rate contrasts with observations on young military servicemen, in whom the re-infection rate was high, thus indicating that the infection with the winter 1977-78 virus had conferred only modest protection against the closely related virus which caused the winter 1978-79 outbreak.