Serum HI antibody and protection against influenza: a follow-up survey at community level of three epidemics caused by different H3N2-variants.

Serial blood specimens from Rh-negative pregnant women sent to laboratory for Rh antibody testing were stored and used for influenza investigations. The study period covered three epidemics, each caused by a different variant of influenza A (H3N2) virus. The relationship between pre-epidemic haemagglutination inhibiting (HI) antibody level against the epidemic virus and serological evidence of infection was analysed. Titre associated with protection was very similar in the three epidemics. In 1973-74 the influenza A epidemic had an exceptional course characterized by prolonged duration. The study revealed data on the rate of infection in one age and sex category that are difficult to obtain using standard indicators of influenza epidemiology.