Abstract: To provide a quality control test for quality of vaccines against a pathogen, a test to determine the presence of antibodies against the pathogen to determine the status of protection of individuals in a herd or a vaccine against the pathogen, in one technique for pathogens having a delayed introduction of protective antibodies, sera from an infected animal is obtained before the eliciting of neutralizing antibodies and after the eliciting of neutralizing antibodies. The two sera are used to remove the non-neutralizing epitopes from the mixture of neutralizing epitopes and non-neutralizing epitopes to obtain pure neutralizing epitopes such as for example by affinity chromatography.