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Polysaccharide (PS) based meningococcal vaccines are primarily evaluated by physicochemical methods to ensure batches are consistently manufactured. As PS content is determined by different methods across numerous laboratories, there is a need for International Standards (IS) to calibrate the assays. Following the successful introduction of the WHO Meningococcal group C (MenC) IS in 2011, NIBSC initiated projects to prepare similar standards for groups A, W, Y and X (MenA/W/Y/X) to standardise all meningococcal- PS based vaccines. On the basis of results from a collaborative study to evaluate preparations of MenA and MenX PS, both were established by the WHO Expert Committee on Biological Standardization in Oct 2015 as; the First WHO International Standard for the Meningococcal Group A polysaccharide with a content of 0.845 ± 0.043 mg MenA PS per ampoule (expanded uncertainty with coverage factor of k=2.45 corresponding to a 95% level of confidence); the First WHO International Standard for the Meningococcal Group X polysaccharide with a content of 0.776 ± 0.089 mg MenX PS per ampoule (expanded uncertainty with coverage factor of k=2.45), as determined by quantitative NMR. The standards are available from NIBSC, who act as guardians and distributors of the material under the auspices of WHO.