Indirect haemagglutination test for the assay of antibodies against Japanese encephalitis viral antigen in human & animal sera.

The indirect haemagglutination (IHA) test was standardized for the assay of antibodies against Japanese encephalitis (JE) virus. Glutaraldehyde fixed sheep erythrocytes were sensitized with purified and concentrated JE vaccine (200-300% brain concentration). The JE vaccine made from Nakayama-NIH strain of JE virus was purified by protamine sulphate treatment and by ultracentrifugation in a sucrose gradient. The sensitized cells were quite stable in liquid as well as in lyophilized state both at -70 degrees C and 4-8 degrees C. These cells could be used for two years without much loss (4-8 times loss) in titre. The IHA test was as sensitive as the neutralization (N) test performed by plaque reduction method in chick embryo fibroblasts. The sensitivity of the test was influenced by the source of erythrocytes i.e., from the different sheep from which these were drawn. After standardization of the test, 16 human sera and 18 sera of immunized mice were assayed for antibodies against JE virus by N and IHA tests. There were no significant differences between titres of both human and mice sera determined by N and IHA tests (P greater than 0.05). The correlation coefficient between N and IHA titres for human sera was 0.60 (P less than 0.05) and for mice sera 0.82 (P less than 0.01). The IHA test has been found to be very simple, inexpensive, sensitive and reproducible.

Immune status of volunteers one year after administration of Japanese encephalitis vaccine produced in India.

The immune status of 40 volunteers who received the full course of Japanese encephalitis (JE) vaccine a year earlier and 15 individuals who had received only a booster dose at the same time, was studied by estimating the level of persistence of protective antibody in the sera. All the sera showed persistence of 100 per cent seroconversion rate. Individuals who had the full course of vaccination still had high levels of antibody (mean 2.8 Iog10); however there was a fall of 0.8 Iog10 from the post-booster level. Volunteers who had received only a booster dose, also showed persistence of high level of protective antibody (mean 2.4 Iog10), a drop of 0.9 Iog10 from the post-booster level. Neutralizing (N) antibody estimated using Dibrugarh (7812474) strain of JE virus also demonstrated persistence of high level of protective antibody against this virus (mean 2.4 Iog10). Persistence of high level of protective antibody against homologus and heterologus (Dibrugarh) virus strains and absence of vaccine related side-effects even one year after administration of JE vaccine produced in India, demonstrates the immunizing potency and safety of this new vaccine.