These diseases were once quite common in the . and led to many deaths. However, routine vaccinations have helped nearly eliminate tetanus and diphtheria infections. Pertussis is the only vaccine-preventable disease that continues to rise in the . Before 2005, only young children could receive the pertussis vaccine . Waning immunity and inadequate vaccination -- many parents choose not to vaccinate their children -- have led to a resurgence of the disease in the . in recent years. Outbreaks of pertussis among adolescents and adults have been reported in several states.
When two or more vaccines are mixed together in the same formulation, the two vaccines can interfere. This most frequently occurs with live attenuated vaccines, where one of the vaccine components is more robust than the others and suppresses the growth and immune response to the other components. This phenomenon was first noted in the trivalent Sabin polio vaccine , where the amount of serotype 2 virus in the vaccine had to be reduced to stop it from interfering with the "take" of the serotype 1 and 3 viruses in the vaccine.  This phenomenon has also been found to be a problem with the dengue vaccines currently being researched, [ when? ] where the DEN-3 serotype was found to predominate and suppress the response to DEN-1, −2 and −4 serotypes.