Abstract
A prospective serological and clinical study of the epidemics due to the A2/Hong Kong/68 influenza virus was made during the winters 1968-9 and 1969-70 in volunteer subjects in the Royal Air Force. In October 1968 nearly all subjects had haemagglutination inhibiting (HI) antibody to the A2/Singapore/57 virus and more than half had antibody to strains more recently prevalent in Britain. The proportion with HI antibody to A2/Hong Kong/68 increased from 31% in October 1968 (most at low titres) to 44% after the first epidemic and 72% after the second (most at high titres). Serological infection rates were much lower in those who had detectable antibody at the beginning of each winter than in those who did not. Respiratory illnesses coupled with serological evidence of influenza infection during the winter were rare in persons with an initial titre of HI antibody of 1/40 or more. Infection in the first winter conferred complete protection against infection, with or without illness, in the second. In both epidemics about half those with serological evidence of infection had no reported illness.