ABSTRACT
57 school children were immunized with the Japanese encephalitis (JE) veccine produced at NIHE, Hanoi (Nakayama vaccine strain). One year after the third dose, their sera were examined for the presence of antibodies against a JE virus clinical isolate (HNH 693, isolated in North VietNam). The results showed that 100% of the serum samples had high antibody titers against both HNH 693 and Nakayama strains. the antibody titer against 693 was significantly higher than thatagainst Nakayama. Potency testing of 11 batches of NIHE-JE vaccine were carried out using parallely HNH 693 and Nakayama as challenge strains. It showed that all these 11 vaccine batches had high potency against both challenge strains (2.62 with HNH 693 and 2.38 with Nakayama).