Notably, for aP combination vaccines, three different WHO international reference standards from the National Institute for Biological Standards and Control (NIBSC), namely, 06/142 [pertussis toxin (PT), filamentous hemagglutinin (FHA), and pertactin (PRN)], 10/262 (diphtheria), and TE-3 (tetanus) are recommended for the calibration of immune assays for the determination of levels of antibodies against pertussis, diphtheria, and tetanus antigens, respectively. This evidence concerns the gene CIAO3 and diphtheria.