Influenza B at Christ's Hospital: natural antibody to influenza B estimated by radial haemolysis.

The technique of radial haemolysis (SRH) was used to assess the response to infection with different strains of influenza B virus, to determine the persistence of antibody following such infection and to examine sera from boys entering school at age 11 years. The technique detected 95% of infections and in primary infection the antibody response was mainly to the infecting strain. Re-infections resulted in a broad response, both to the homotypic strain and to strains more distantly related. Antibody to the homotypic strain persisted for at least 3 years but in some individuals the reaction with heterotypic strains tended to become weaker--resulting in zones of incomplete lysis--or was lost. Examination of the sera collected on entry to the school showed that about 60% of the boys bled before B/Hong Kong became widespread in the United Kingdom had antibody to strains representative of those isolated in the 1960s and few boys had antibody to B/Hong Kong. After 1974 antibody to B/Hong Kong and later strains became more common while antibody to earlier strains was less frequently detected. The significance of the results as an estimate of past experience is discussed.