Why should we not give PCV vaccines to someone who has had a serious reaction to a diphtheria-containing vaccine in the past?

PCV vaccines are conjugated to a type of diphtheria toxoid. If someone has a past history of anaphylaxis following diphtheria-containing vaccine, it might be due to the diphtheria toxoid, and the cause of the anaphylactic allergy should be identified before the administration of a PCV vaccine. This could be difficult since no single-antigen diphtheria toxoid is available in the U.S. Fortunately, true anaphylactic allergy to diphtheria-containing vaccine is rare.