AbstractLive varicella vaccines are known to provide robust immunity against varicella zoster virus (VZV) infections. However, problems with viral attenuation have led to pathogenic VZV vaccine strains causing varicella‐like rash and herpes zoster in immunocompetent children after immunization. We report the first fatal case of VZV infection caused by OKA/SK strain contained in the vaccine administrated as a booster shot in an immunocompetent child, which has been independently developed from any currently available varicella vaccines that are OKA strain or MAV/06 strain based. The patient died due to sudden pulmonary alveolar hemorrhage as a secondary complication of VZV pneumonitis. Sequencing of the four SNPs unique to the OKA/SK strain (SNP loci 14 035T; 32 626C; 58 777G; 70 319G) enabled discrimination of the strain responsible for the disseminated infection. OKA/SK strain does not have any SNPs in ORF62 postulated to be responsible for the attenuation of varicella vaccines which have been safely and effectively used world‐wide or locally, and exclusively enriches a virulent factor in ORF31 identified in parental OKA strain, thus possibly resulting in disseminated VZV infection leading to mortality. Therefore, actions need to be taken to prevent vaccine related morbidity and mortality in children.