Haemagglutination-inhibition (HI) and virus neutralisation (VN) assays are routinely applied to evaluate influenza vaccine immunogenicity for regulatory approval. Despite their frequent use both assays are currently only poorly standardised causing considerable inter-laboratory variation of serological results that is particularly evident for pandemic influenza vaccines. The present study was conducted in association with the European Medicines Agency (EMA) to directly compare assay variability between vaccine manufacturer's and European regulatory agency's laboratories in an influenza pandemic scenario. To this end, a defined subset of H1N1 pdm clinical trial sera from all manufacturers that had applied at EMA for approval of pandemic H1N1 vaccines were re-tested by the National Institute for Biological Standards and Control (for HI) and the Paul Ehrlich Institute (for VN). Comparative analysis of test results determined for almost 2000 serum samples revealed a marked inter-laboratory variation for HI titres (up to 5.8-fold) and even more for neutralisation titres (up to 7.0-fold). When the absolute titres were adjusted relative to the calibrated International Antibody Standard 09/194 variation was drastically reduced and acceptable agreement of results from different laboratories could be achieved. Hence, inclusion of an appropriate calibrated antibody standard for adjustment of original titres is a powerful tool to substantially increase reproducibility of serological results from different laboratories and to significantly improve regulatory evaluation of influenza vaccine efficacy.
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