The CDC said it, not me:
Before childhood vaccination was introduced in the 1940s, pertussis was a major cause of infant death worldwide. Widespread vaccination of children succeeded in reducing illness and death. In the 1990s, a resurgence of pertussis was observed in a number of countries with highly vaccinated populations, and pertussis has become the most prevalent vaccine-preventable disease in industrialized countries. We present evidence that in the Netherlands the dramatic increase in pertussis is temporally associated with the emergence of Bordetella pertussis strains carrying a novel allele for the pertussis toxin promoter, which confers increased pertussis toxin (Ptx) production. Epidemiologic data suggest that these strains are more virulent in humans. We discuss changes in the ecology of B. pertussis that may have driven this adaptation. Our results underline the importance of Ptx in transmission, suggest that vaccination may select for increased virulence, and indicate ways to control pertussis more effectively.'
Also this from the Center for infectious disease dynamics - both conventional medical sources:
An acellular whooping cough vaccine actually enhances the colonization of Bordetella parapertussis in mice; pointing towards a rise in B. parapertussis incidence resulting from acellular vaccination, which may have contributed to the observed increase in whooping cough over the last decade.
Read that before you start commenting - I only state what has been formally researched and discovered, and as the above source says, they have discovered it DOES mutate into parapertussis in mice tests. Animal testing is the medical model of safety for drugs.