On 2017 Jun 21, Gary Goldman commented:

It is well established that in Japan, where only 1 in 5 children received varicella vaccination, that those vaccinated maintained protective immunity for 20 years. This was largely due to the vacinees' receiving exogenous exposures to children with natural or wild-type varicella that boosted cell-mediated immunity to varicella zoster virus. During the majority of the study by Kuter et al., varicella vaccine receipients were likewise being boosted through exposure to individuals with wild-type varicella--especially during annual epidemics of varicella. This had the effect of artifically elevating vaccine effectiveness because of the shedding of varicella zoster virus in the community. In communities with widespread varicella vaccination, once oppotunities for exogenous boosts became rare, single-dose varicella vaccination efficacy declined to less than 50% five years after vaccination.Goldman GS, 2005

On 2017 Jun 21, Gary Goldman commented:

It is well established that in Japan, where only 1 in 5 children received varicella vaccination, that those vaccinated maintained protective immunity for 20 years. This was largely due to the vacinees' receiving exogenous exposures to children with natural or wild-type varicella that boosted cell-mediated immunity to varicella zoster virus. During the majority of the study by Kuter et al., varicella vaccine receipients were likewise being boosted through exposure to individuals with wild-type varicella--especially during annual epidemics of varicella. This had the effect of artifically elevating vaccine effectiveness because of the shedding of varicella zoster virus in the community. In communities with widespread varicella vaccination, once oppotunities for exogenous boosts became rare, single-dose varicella vaccination efficacy declined to less than 50% five years after vaccination.Goldman GS, 2005