Did you know that people who have been vaccinated against diphtheria can still carry and spread diphtheria without showing any symptoms of infection?

“There was no statistical difference in risk of diphtheria infection among those who had complete, expired, inadequate, or no prior vaccination once morest diphtheria … Diphtheria toxoid … does not prevent carrier status or stop the spread of infection Identification, isolation and treatment of.” Carriers are very important aspects in fighting diphtheria epidemics.”

Those: https://www.ncbi.nlm.nih.gov/pubmed/5026197