Since Hong Kong influenza appeared in the fall of 1968, and was found to fulfill the requirement as a new variety with antigen structure different from that of A
2 type, studies conducted during the epidemic of A
2 type were extended in search of this new type at various fileds. The author took this opportunity to study the relationship between the degree of resistance of the group and epidenic from the viewpoint of immunoserology. The results may be summarized as follows : 1. The prevalence of patients with apparently no resistance in each group, those with HI titer of less than 16 and above 4, was slightly different according to the resistance of the group. This is probably due to the fall of the potency of transmission of influenza in a group with high rate of antibody maintenance leading to a fall of the potency of infection. 2. The frequency of affection or rise of HI titer to more than 4 was the highest in cases with HI titer below 16, (58.5%). This dropped markedly in cases with HI titer of 64, to only 2.1%. 3. The state of prevalence of the group or the frequency of rise of HI titer above 4 was higher as the proportion of cases with HI titer 32 and below became greater. This would indicate that the resistance of group is determined by the proportion of cases with HI titer above 64. 4. The preventive effect of vaccine is determined by the provision of antibody titer above 64. Hong Kong influenza is at present said to be slower in spread than A
2 type. Our results also indicated lower antibody titer after infection. This also suggests the possibility of repeated epidemic by this virus.
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