A standardised assay for the evaluation of typhoid vaccines

Scientists from the National Institute for Biological Standards and Control have led a multi-laboratory study to evaluate a standardised assay for determining the potency of typhoid conjugate vaccines.

The results of this study, funded by the Bill and Melinda Gates Foundation, were recently published in the journal Biologicals. It is hoped that the use of this assay will help to accelerate the licensing process for vaccines that are undergoing clinical trial.

Typhoid is a bacterial infection that represents a major challenge for public health, particularly in developing countries. Vaccination remains the leading strategy for preventing disease outbreaks and it is known that typhoid conjugate vaccines (TCVs) offer longer lasting protection than conventional typhoid vaccines and are also safe enough for use with infants. At present, there are three TCVs licensed for use with several more in clinical and pre-clinical development.

The Enzyme-linked immunosorbent assay, or ELISA, is a commonly used laboratory method for assessing the immunogenicity of typhoid vaccines. It is used to measure antibodies that bind to the Vi polysaccharide capsule, which envelopes the typhoid bacterium and is responsible for its virulence. Previous work has revealed that ELISAs based on this Vi antigen can sometimes produce inconsistent results, most likely due to the fact that the Vi polysaccharide binds poorly to the plastic plate used in the assay. However, the International Vaccine Institute of Seoul previously developed a solution to this problem by coating ELISA plates with a poly-amino acid chain called poly-L-lysine (PLL), which enhances the binding of Vi polysaccharide to the plate surface.

In this new study, the performance of the Vi-PLL ELISA was evaluated by ten different laboratories from seven countries using sera from volunteers immunised with licensed typhoid vaccines. The potency of the vaccines was expressed relative to the International Standard for anti-Vi IgG (human). Crucially, the results revealed that the Vi-PLL ELISA demonstrated good repeatability and reproducibility and can be used as a credible alternative to an existing commercial ELISA kit.

Dr Sjoerd Rijpkema, who led the study, comments that:

“With the Vi-PLL ELISA, scientists worldwide can now freely access a fully documented procedure of a reliable assay to study the serology of typhoid vaccines. The use of this standardised assay will enable the harmonisation of clinical trial data meaning that effective vaccines can be licensed and placed on the market sooner.”

Further information about our typhoid research can be found here.