Influenza epidemic was observed in Hiroshima Prefecture in 1973 with peaks in May and November. The incidence and levels of hemagglutination inhibition (HI) antibodies to influenza A and B virus strains in the sera of patients, the sera of healthy persons and sera collected prior to the epidemic were compared.
HI antibodies to B/Hong Kong/5/72 and B/Hiroshima/9/73 (closely related to B/Hong Kong/5/72) were recognized in patient sera and although significant rise in HI titers was observed in paired sera from patients, HI antibodies to these viruses were not observed in sera collected from healthy persons during the epidemic and prior to it.
Geometric mean titers of HI antibodies in convalescent sera to B/Hong Kong/5/72 type viruses were significantly lower than those to B/Kanagawa/1/73 and also preexisting B viruses. The distribution range of HI titers to B/Hong Kong/5/72 type viruses in convalescent sera of the group of 1973 spring patients, the group of 1973 autumn-winter patients not vaccinated with 1973 influenza-vaccine and the group of 1973 autumn-winter patients who received the primary dose of 1973 vaccine after recovery were mainly 1: 16-1: 32, 1: 32-1: 64 and 1: 32-1: 128, respectively, and there were no sera with HI antibody titer exceeding 1: 512. It is considered that this is because there was no infection of this type of virus nor vaccine-stimulation until the epidemic of 1973.
As a result of estimation of HI antibody possessing rate and distribution of HI titers, it was concluded that the influenza epidemic of 1973 was due chiefly to B/Hong Kong/5/72 type virus and it was further made clear that outbreaks due to B/Hong Kong/5/72 type virus occurred in May-July, 1973 in Hiroshima Prefecture when B/Kanagawa/1/73 type viruses were isolated.