Researchers from the University of Colorado have developed mutated viral strains that only replicate under defined conditions and can be used as shingles vaccines. The inventors found that ORF 63 or 70 protein expression is required for shingles virus replication. They developed a viral mutant in which ORF 63 is deleted and ORF 70 protein is typically degraded, but in the presence of a specific antibiotic, it is stable and promotes viral replication. These results suggest that a vaccine comprising of such conditional mutants administered with the antibiotic will elicit a protective immune response against the virus but that replication of the virus can be reversibly completely “turned off” by the removal of the antibiotic.