Résumé
Efficacy of mouse brain inactivated Japanese encephalitis vaccine was evaluated by studying the immune status of volunteers 1,2,3,4.5 and 7.5 yr after immunization. Neutralizing (N) antibody which is protective and found to correlate with the immunity after vaccination was estimated in serum by plaque reduction neutralizing test on chick embryo cell monolayer. Mean N-antibody titres of 3.25 (pre-booster) and 3.6, 2.8, 2.06, 1.85 and 1.50 log10 were observed post-booster, and 1,2,3, and 4.5 yr of immunization in volunteers who received complete immunization (3 doses). All the volunteers retained more than 1.0 log10 titre of protective N-antibody in spite of the loss of 0.8, 0.74, 0.21 and 0.35 log10 after 1,2,3, and 4.5 yr respectively. Similarly mean N-antibody titres of 1.6, 3.25, 2.4, 2.25, 1.92 and 1.60 log10 were observed pre-booster, after a single booster dose, and 1,2,3 and 4.5 yr of vaccination in individuals who received only a single booster dose. Ten serum samples of volunteers tested after 7.5 yr of vaccination showed that those who were in constant contact with JE virus (n = 7) in the laboratory maintained high levels of N-antibody whereas others (n = 3) showed a fall in titre indicating the necessity of a booster dose.