Immunogenicity of the AIK-C measles vaccine in infants aged <9 months in Vietnam.

Measles-associated deaths have been reported in infants <9 months during outbreaks. A cohort study was conducted on 210 infants aged 6-8 months to evaluate the immunogenicity and safety of the AIK-C measles vaccine containing 104.21 plaque-forming units (PFU)/0.5 mL produced in Vietnam. Paired serum samples were obtained from 196 subjects. Seropositivity was defined as ≥120 mIU/mL. The seroresponse rate was 173/196 (88.27%, 95% confidence interval (CI): 83.77-92.77%) with geometric mean titer (GMT) of 511 mIU/mL (95% CI: 688-880 mIU/mL), and no significant differences were observed by different age groups. Among 196 paired sera, they were categorized into four groups: 122 subjects <14 IU/mL, 28 subjects 14-<60 mIU/mL, 30 subjects 60-<120 mIU/mL, and 16 subjects ≥ 120 mIU/mL. The seroresponse rate was 112/122 (91.8%, 95% CI: 86.94-96.67%) with GMT (597 mIU/mL, 95% CI: 749-1002 mIU/mL) in the <14 mIU/L group. In the 14-<60 mIU/mL group, the seroresponse rate was 18/28 (64.29%) with 184 mIU/L of GMT and was significantly lower (p < 0.01) than that in the <14 mIU/mL group. In the 16 seropositive group, all subjects showed seroconversion (4-fold higher than before) with a higher GMT of 1078 mIU/mL. Local pain and itching at the injection site were observed in 8 subjects (3.8%) within 7 days of the vaccination. Regarding systemic adverse reactions, febrile illness ≥37.5 °C was observed in 14 subjects (6.7%). These results indicate that the AIK-C measles vaccine is effective and safe for infants aged 6-8 months and will contribute to reducing the number of measles-associated deaths in future outbreaks.