The present invention relates to a method for purifying a virus, or a viral antigen thereof, comprising at least the following steps: a) obtaining a fluid comprising the virus, or a viral antigen thereof, and b) purifying the fluid by at least one density gradient ultracentrifugation step, wherein the ratio of the amount of virus, or viral antigen thereof, present in the fluid over the density gradient volume is less than 1, less than 0.8, less than 0.6 and less than 0.4.