Shingrix In Recipients of Allogeneic Transplants

Inclusion Criteria: * Allo-SCT recipients being age 18 - 79 years at time of allo-SCT. * Written informed consent being obtained from the subject * Two doses of RZV, separated by 2 to 6 months, administered at least 1 year after allo-SCT. * Enrollment at \>/= 18 months after second dose of Shingrix. * Female subjects of childbearing potential (FOCBP) enrolled in the study only if they: * have practiced adequate contraception for 30 days prior to vaccination with any dose of zoster vaccine and * have a negative pregnancy test on the day of each dose of zoster vaccine and * agree to continue adequate contraception during the vaccination period and for 2 months after receipt of the vaccine. * Investigator belief that the participant will comply with the requirements of the protocol Exclusion Criteria: * Active Graft Versus Host Disease (aGVHD) at the time of enrollment and receipt of the third dose of RZV * Having received ≥20 mg prednisone for more than 2 weeks (or equivalent) in the 8 weeks preceding enrollment. * Receiving any significant immunosuppressive therapy other than for graft maintenance, in the opinion of the investigator. * Having received a live attenuated vaccine within the last 4 weeks, or inactivated vaccine in the last 2 weeks, prior to enrollment. * Having a history of HZ after the administration of the primary 2-dose RZV immunization regimen. * Pregnancy or breastfeeding * Receiving investigational drugs from 30 day before enrollment or planned during the study * Inability of participants unable to comply with the study schedule in the opinion of the investigator