Vaccines (Apr 2024)

Development and Validation of a Plaque Assay to Determine the Titer of a Recombinant Live-Attenuated Viral Vaccine for SARS-CoV-2

  • Einat Toister,
  • Lilach Cherry,
  • Edith Lupu,
  • Arik Monash,
  • Eyal Dor,
  • Lilach Levin,
  • Meni Girshengorn,
  • Niva Natan,
  • Shira Chapman,
  • Shlomo Shmaya,
  • Eyal Epstein,
  • Yaakov Adar,
  • Ran Zichel,
  • Yakir Ophir,
  • Eran Diamant

DOI
https://doi.org/10.3390/vaccines12040374
Journal volume & issue
Vol. 12, no. 4
p. 374

Abstract

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The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has resulted in more than seven million deaths worldwide. To reduce viral spread, the Israel Institute for Biological Research (IIBR) developed and produced a new rVSV-SARS-CoV-2-S vaccine candidate (BriLife®) based on a platform of a genetically engineered vesicular stomatitis virus (VSV) vector that expresses the spike protein of SARS-CoV-2 instead of the VSV-G protein on the virus surface. Quantifying the virus titer to evaluate vaccine potency requires a reliable validated assay that meets all the stringent pharmacopeial requirements of a bioanalytical method. Here, for the first time, we present the development and extensive validation of a quantitative plaque assay using Vero E6 cells for the determination of the concentration of the rVSV-SARS-CoV-2-S viral vector. Three different vaccine preparations with varying titers (DP_low, DP_high, and QC sample) were tested according to a strict validation protocol. The newly developed plaque assay was found to be highly specific, accurate, precise, and robust. The mean deviations from the predetermined titers for the DP_low, DP_high, and QC preparations were 0.01, 0.02, and 0.09 log10, respectively. Moreover, the mean %CV values for intra-assay precision were 18.7%, 12.0%, and 6.0%, respectively. The virus titers did not deviate from the established values between cell passages 5 and 19, and no correlation was found between titer and passage. The validation results presented herein indicate that the newly developed plaque assay can be used to determine the concentration of the BriLife® vaccine, suggesting that the current protocol is a reliable methodology for validating plaque assays for other viral vaccines.

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