Immunogenicity and Safety of a Newly Developed Live Attenuated Varicella Vaccine in Healthy Children: A Multi-National, Randomized, Double-Blinded, Active-Controlled, Phase 3 Study

Heightened incidence of adverse events associated with a live attenuated varicella vaccine strain that lacks critical genetic polymorphisms in ORF62

OBJECTIVES

This study aimed to identify the specific vaccine strain associated with herpes zoster in children following a series of diagnosed cases and to explore whether differences in single nucleotide polymorphisms (SNPs) among various vaccine strains are linked to an increased incidence of herpes zoster after vaccination.

METHODS

From February 2021 to March 2024, children <12 years old suspected of vaccine-related varicella-like rash or HZ were included. Varicella zoster virus DNA isolated from the patients were sequenced to differentiate vaccine type versus wild type. 3D protein structures of pORF62 were simulated using ORF62 sequences extracted from whole genome sequencing of vOka, MAV/06, Oka/SK vaccines, and pOka reference.

RESULTS

A total of 27 children with a median age of 2.1 (IQR, 1.5-3.4) years old presented with vaccine-related varicella-like rash (n=4/27, 14.8%) or HZ (n=23/27, 85.2%). One patient with varicella-like rash and 34.8% (n=8/23) with HZ had disseminated skin involvement. All were immunized with the Oka/SK strain varicella vaccine. Genotyping showed 88.2% (n=15/17) had SNPs specific to the Oka/SK strain and two had SNPs considered pOka type contained within the Oka/SK vaccine. Despite accumulations of SNPs in ORF 62 of Oka/SK, the translated amino acid sequence and 3D protein structure were identical to wild-type pOka's pORF62. In vOKA and MAV/06, changes in amino acids occurred at two positions, S628G and R958G, within pORF62. The predicted 3D protein structure of vOka and MAV/06's pORF62 showed that the alpha helical structure within region I undergoes conformational change potentially increasing difficulties in interactions with infection-related proteins thereby decreasing virulence. pORF62 in pOka and Oka/SK exhibited more stable structure complex of the alpha helical structure.

CONCLUSIONS

Lack of structural alternations in region I of pORF62 due to the absence of critical genetic polymorphisms in ORF62 could be associated with the heightened incidence of adverse events.

A booster administration of the OKA/SK strain causes fatal disseminated varicella in an immunocompetent child

Live 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.