A long-term observation of HI antibody against Japanese encephalitis virus and the effect of vaccination on the immunity of healthy adults

Abstract

A long-term observation on HI antibody against Japanese encephalitis virus (JEV) was performed in a group of healthy adults extending from 1966 to 1982.The results obtained are as follows:1. In June 1967, 224 HI antibody negative persons were vaccinnated twice by subcutaneous injection of 1.0 ml of JEV vaccine, and 178 persons (79.5%) changed to seropositive a month after the first inoculation. Titers of HI antibody of 138 persons (77.5%) ranged from 1: 10 to 1: 40. The levels of the serum HI antibody acquired after vaccination were not maintained for long term for the most cases; 80% of them returned to seronegative one year after. In a few cases, they returned to seronegative from as high titer as 1: 160.2. In 1971, 106 persons showed the positive HI test at the titer of 1: 10, and they received a single dose of vaccine in 1972. In 1975, 65 persons (61.3%) were seronegative, among which 52 persons received one or two doses of vaccine, and 50 of them (96.2%) acquired the detectable HI antibody again at titers of 1: 10 to 1: 80.3. Although 220 persons who showed the HI titer of 1: 20 or higher in 1971 did not received the booster vaccination for the following six years, 193 of them (87.7%) were still seropositive in 1975. All of the persons who showed the positive HI test at titers of 1: 80 or higher in 1971 maintained detectable HI antibody for a period of six years. At least, none of 51 persons were vaccinated since 1966, and all of them maintained HI antibody at titers of 1: 10 to 1: 320 for sixteen years. It seems that these persons probably acquired the antibody after the subclinical infectioin with JEV and HI antibody was maintained for a long term in these cases.4. After fifteen years of the systematic vaccination against JEV as mentioned above, positive rate of HI antibody in the group increased from 43.4% in 1966 to 67.5% in 1971 and 73.2% in 1982, respectively. In a period of the observation, outbreaks of Japanese encephalitis were endemic in Japan, and it seems that the upturn of the rates of the seropositives is due to the effect of vaccination and not to the immunization by natural infection of JEV.5. One hundred and one persons who showed negative HI test in 1966 were frequently inoculated with JEV vaccine thereafter, but 38 out of 101 were still seronegative and 33 showed low HI antibody titer (1: 10) in 1982. The rate of such low responders was estimated at about 14% of all of the members in this series. No linkage was observed between the low responsiveness to JEV by HI test and the certain types of HLA-A, B, and C antigens. Most of these HI antibody negative cases showed positive neutralizing antibody. It is suggested that the discrepancy between HI test and neutralization test is not due to the difference of sensitivity of these tests, but may be due to the constitution of individuals.