The hemagglutination inhibition antibody responses to an inactivated influenza vaccine among healthy adults: With special reference to the prevaccination antibody and its interaction with age

The immunogenicity of the trivalent split-virus influenza vaccine was investigated among 70 healthy adults (mean age: 48.5, ranger 36-68). The vaccine antigens were: A/Yamagata/32/89 (H1N1); A/Beijing/352/89 (H3N2), and B/Bangkok/163/90. Regarding the entire sample, the vaccine induced a tenfold or more rise on the average in the hemagglutination inhibition (HAI) antibody to each antigen. The response rates (greater than or equal to a fourfold rise) were about 90% or more among those with a prevaccination titer less than or equal to 1:64 (equivalent to less than or equal to 1:16 on the Western scale: in Japan, the HAI titers are expressed by the final, and not the initial, dilution of the serum, from hereon our findings will be expressed using the Japanese scale), whereas they were 0-50% at greater than or equal to 1:128. Thus, the prevaccination titer was negatively associated with antibody induction. The achievement rates (postvaccination titer greater than or equal to 1:128) among those with a prevaccination titer < 1:16 remained at 48-68%. Regarding the analysis of variance, a significant effect on antibody induction was indicated for the prevaccination titer (P less than or equal to 0.002), but not for age (P greater than or equal to 0.425). The interaction between the prevaccination titer and age was significant for A/Yamagata (P=0.030), while it was also suggestive for A/Beijing (P=0.054). as age increased, those with no preexisting antibody (<1:16) showed greater titer rises, in contrast to the smaller rises among those with a titer greater than or equal to 1:16. Based on the attack survey conducted separately, the vaccine efficacy on influenza-like illnesses with fever greater than or equal to 37 degrees C and greater than or equal to 37.5 degrees C was calculated to be 16% (95% confidence interval: -66% to 57%) and 37% (-55% to 74%), respectively.