[A study for the necessity of virus titer of varicella vaccine presently used].

Recently available varicella vaccine titers are several dozen times higher than the formulation standard in accompanying information, i.e., > or = 1,000 PFU/dose. We investigated changes in immunogenicity associated with vaccination using a reduced dose whose virus titer was close to that used when the vaccine was developed, and examined the need for the virus titer presently used. In a non blinded study of 43 children with no history of varicella infection, we administered 0.1 mL of varicella vaccine (1/5 of the normal dose) to 20 children 1 year 0 month to 4 years 5 months old (median: 1 year 5 months) and the standard 0.5mL dose to 23 children 1 year 2 months to 3 years 7 months old (median: 1 year 9 months). We measured IAHA and gpELISA antibody titer before vaccination and 4 to 6 weeks after vaccination. We defined "positive" as > or = 2 fold of IAHA titer and > or = 50 U of gpELISA antibody titer. We administered an additional 0.5mL of varicella vaccine to children whose IAHA titer failed to show seroconversion and remeasured antibody titer 4 to 6 weeks after revaccination. IAHA seroconversion was 25.0% (5/20) and gpELISA seroconversion 55.0% (11/20) in the 0.1 mL vaccination group, which was lower than that of 76.2% (16/21) and 87.0% (20/23), IAHA p < 0.01, gpELISA p < 0.05, in the 0.5 mL vaccination group. We administered an additional vaccination to 19 children--15 in the 0.1 mL vaccination group and 4 in the 0.5 mL vaccination group-with 100% seroconversion for both methods. Mean antibody titer after revaccination in the 0.1 mL vaccination group (IAHA 2 (6.0), gpELISA 10 (3.7)) was higher than those in the 0.5mL vaccination group who seroconverted following initial vaccination (IAHA 2(4.5), gpELISA 10(2.6)) (p < 0.01). We also measured virus titer in the remaining vaccine following vaccination of 0.1 mL (n = 20), and estimated virus titer administered to the 0.1 mL vaccination group to be 2,600-6,400 PFU/ dose. Varicella vaccine immunogenicity decreased if dosage was reduced to 1/5 of the standard dose, indicating that the present virus titer is necessary to maintain adequate immunogenicity. An additional administration of the standard dose to children who failed to seroconvert after initial 0.1 mL administration produced high antibody titers thought to constitute a booster effect.