Viral protein 2 and viral protein 3 (VP2 and VP3) were quantified in a series of inactivated infectious bursal disease oil emulsion vaccines using enzyme-linked immunosorbent assay, and the dependence of the serological response on vaccine antigen content was studied. Large differences in antigen content, up to 50-fold, were found between vaccines. Neutralizing antibody titres at 3 to 6 weeks after vaccination varied from 3 log 2 to 16 log 2 . None of the vaccines induced an antibody titre

more »

... to that of the reference serum used as an indicator of sufficient potency in the European Pharmacopoeia. Neutralizing antibody titres after vaccination correlated highly with the VP2 content of the vaccines. A significant correlation was also found between the VP3 content and the antibody response. Our data illustrate that the antigen content of inactivated infectious bursal disease vaccines is a reliable indicator of the protective serological response after vaccination, and consequently could be used as a measure of vaccine potency. This holds true for both VP2, the antigen that induces neutralizing antibodies, as well as for VP3, which does not induce neutralizing antibodies.