- Young adults aged 25-40 years who are Varicella-Zoster virus seronegative or equivocal results (mean value OD for Varicella-Zoster virus IgG lower than 1.1 by commercial enzyme-linked immunosorbent assay (ELISA) (Hope Clinic: IgG VZV ELISAII-Wampole Laboratories®, NJ, USA- Vaccine Research Trials Center:: Liaison VZV IgG Assay, DiaSorin, Italy)