In anticipation of large-scale
smallpox vaccination, clinical trials of new
vaccine candidates with improved safety profiles, and new
vaccinia immune globulin (VIG) products, there is an immediate need to develop new assays to measure
vaccinia-specific immune responses. The classical assay to measure
vaccinia neutralization, the plaque-reduction neutralization test (PRNT), is slow, labor intensive, and difficult to validate and transfer. Here we describe the development of a novel
vaccinia-neutralization assay based on the expression of a reporter gene,
beta-galactosidase (beta-Gal). Using a previously constructed
vaccinia-beta-Gal recombinant virus, vSC56, we developed a neutralization assay that is rapid, sensitive, and reproducible. The readout is automated. We show that the neutralizing titers, ID(50), for several VIG products measured by our assay were similar to those obtained by PRNTs. A new Food and Drug Administration VIG standard was established for distribution to other laboratories. The new assay will serve as an important tool both for preclinical and clinical trials of new
smallpox vaccines and for evaluation of therapeutic agents to treat
vaccine-associated adverse reactions.