To investigate the efficacy and safety of three different doses of ASP2151, as compared to valacyclovir hydrochloride in subjects with herpes zoster, and to determine the recommended clinical dose.

Inclusion Criteria:

• Subjects aged from 20 years to under 80 years on the day informed consent is obtained
• Subjects to whom the study drugs can be orally administered within 72 hours after the onset of rash due to herpes zoster
• Subjects in whom protocol-specified observations and assessments are considered possible

Exclusion Criteria:

• Subjects with extremely compromised immune function due to underlying diseases, treatment with Immunosuppressive agents etc., or radiotherapy
• Subjects with a serious underlying disease that corresponds to Grade 3 in the Classification Criteria for teh Seriousness of Adverse Drug Reactions
• Subjects with concurrent malignant tumors, or those with a history of malignant tumors within the past 5 years

Access to anonymized individual participant level data will not be provided for this trial as it meets one or more of the exceptions described on www.clinicalstudydatarequest.com under "Sponsor Specific Details for Astellas."