We previously reported the short-term immunogenecity and safety of 47 passaged Oka strain live attenuated varicella vaccine in healthy children in 1997. Now, we conducted this two-year follow-up study to confirm the maintenance of immunity, the occurrence of natural varicella infection and the activation of vaccine induced latent infection on the same vaccine.

99 children who had been immunized by 47 passaged Oka strain live attenuated varicella vaccine in 1997 were followed up by questionnaire, and 46 children out of study group were followed up serologically. They were asked to report any instance of varicella or herpes zoster since they had been immunized. If there was any evidence of varicella or herpes zoster, they should be clinically or serologically confirmed by doctor. Also, those patients parents were asked to report any instance of varicella or herpes zoster in their family, playmate, kindergarten, school, or other settings. The immunity to VZV was confirmed by EIA and FAMA test.

6 recipients developed breakthrough varicella after exposure to VZV in family, kindergarten and school during follow-up period. However, clinical features of those patients were very mild and self limited without therapy. And none of the recipients developed herpes zoster during this observation period. The results of EIA test showed that study subjects were all seropositive except one, and the antibody titers and GMT of FAMA test were seropositively maintained in all subjects. Statistically, the antibody titers of EIA and FAMA test confirmed two years after vaccination were higher than those results confirmed one month after vaccination.

Our study results suggest that the immunity of 47 passaged Oka strain live attenuated varicella is well maintained until 2 years later after vaccination, and mild natural infection after exposure to VZV can be occurred with low rate. There were not developing zoster in study vaccine after vaccination for two-years.