Diphtheria disease is caused by the diphtheria toxin, which is produced and secreted by toxigenic C. diphtheriae bacteria. An inactivated form of the diphtheria toxin, called toxoid, constitutes the diphtheria vaccine and is part of DTaP/Tdap. There is no evidence and no biologically plausible mechanism that vaccination with the diphtheria toxoid reduces the risk of person-to-person transmission of toxigenic C. diphtheriae, since the vaccine-induced antibodies are directed only against the toxin and not against whole bacterium. The diphtheria vaccine can only prevent toxin-related symptoms of diphtheria. It does not prevent infection/colonization of the throat by toxigenic C. diphtheriae in the vaccinated. In fact, both vaccinated and unvaccinated children were infected with C. diphtheriae at the same rate in a Texas outbreak of diphtheria in the 1970s, as determined by Miller et al. 1972. “These outbreaks, the known importance of carriers in the spread of diphtheria, and the demonstrated failure of toxoid to prevent the carrier state lead us to conclude that the concept of herd immunity is not applicable in the prevention of diphtheria. A high level of community immunization will not stop the transmission of diphtheria…” (Miller et al., American Journal of Diseases of Children 1972).
 Miller LW, Older JJ, Drake J, Zimmerman S. Diphtheria immunization. Effect upon carriers and the control of outbreaks. Am J Dis Child. 1972 Mar;123(3):197-9. PubMed PMID: 5026197.