The present disclosure relates to a method for determining the potency of an antigen sample such as a vaccine antigen sample. The present disclosure is also related to a method for monitoring the potency of a vaccine antigen during the production process including purifying, inactivating and formulating the vaccine antigen and to a method for producing a virus vaccine. Further, the present disclosure relates to vaccines obtainable by the methods disclosed. In certain embodiments of the present invention the antigen sample is a zika virus antigen sample.