Varivax and Varilrix Strain Comparison
Yes, both Varivax and Varilrix contain the same Oka strain of varicella-zoster virus (VZV), though they are not genetically identical due to different manufacturing passages. 1
Shared Origin
- Both vaccines are derived from the original Oka strain of VZV, which was isolated in Japan in the early 1970s from vesicular fluid of a healthy child with natural varicella 1
- The original Oka strain was attenuated through sequential propagation in cultures of human embryonic lung cells, embryonic guinea-pig cells, and human diploid cells (WI-38) 1
Manufacturing Differences
- Varivax (Oka/Merck): The virus underwent further passage through human diploid-cell cultures (MRC-5) for a total of 31 passages 1
- Varilrix (Oka-V/GSK): Manufactured by GlaxoSmithKline Biologicals with its own additional passages 2, 3
Genetic Similarity
- Complete genome sequencing reveals that Varilrix and Varivax genomes are composed of 124,821 bp and 124,815 bp, respectively 2
- There are only 36 nucleotide differences between the two vaccine strains throughout the entire genome 2
- Among these differences, only 14 result in unique amino acid substitutions 2
- Despite not being genetically identical, the vaccines are very similar, which supports clinical data showing both are well tolerated and elicit strong immune responses 2
Clinical Implications
- Both vaccines contain live-attenuated Oka strain VZV and are used for routine varicella vaccination 4
- Both vaccines have demonstrated similar safety profiles and immunogenicity in clinical practice 5, 2, 3
- The vaccines are considered interchangeable for practical clinical purposes, as the minor genetic differences do not translate into clinically significant differences in safety or efficacy 2
Important Caveat
While both vaccines share the same ancestral Oka strain, they should not be considered absolutely identical products due to the different manufacturing processes and resulting minor genetic variations 2. However, these differences are minimal and do not affect clinical decision-making regarding vaccine selection or interchangeability 2.