Seroprevalence of Neutralizing Antibodies Against Japanese Encephalitis Virus Among 6 Years Old Children With 5 Different Immunization Strategies in Zhejiang Province, China: An Open-labeled, Ambidirectional Cohort Study

Based on the retrospective registry in Zhejiang Provincial Immunization Information System, healthy children aged 6 years from 5 cohorts were included according to their previous immunization schedules on JE: 3 doses of JEV-I (Group 1), JEV-L+ JEV-I (Group 2), JEV-L+ JEV-I + JEV-I (Group 3), JEV-I + JEV-I+ JEV-L (Group 4), and 2 doses of JEV-L (Group 5). According to immunization programs in China, a booster dose of JEV-I is needed at the age of 6 for Group 1 and Group 2. A pre-vaccination blood sample (2.5ml) will be collected to evaluate the proportions of seropositivity before the booster dose of JEV-I. A second blood sample will be collected 28-35 days after the booster dose. Immunogenicity of the booster dose will be assessed by seroconversion rates, proportions of seropositivity and GMTs for these two groups. In addition, safety will be assessed with the occurrence of adverse events reported in these two groups. For the rest three groups who have completed the immunization schedules on JE, only one blood sample will be taken at the age of 6 to evaluate the proportions of seropositivity and GMTs after 4 years post the complete immunization schedules. Plague reduction neutralization test (PRNT) against both vaccine strains (SA-14-14-2 strain for JEV-L and Beijing P3 strain for JEV-I) will be used to test neutralizing antibody titers, so as to avoid potential bias in favor of either vaccine. PRNT50 titers (the reciprocal of the serum dilution that reduced the virus plaque count by 50% compared with the virus-only controls) ≥ 1:10 are deemed as positive.