I think you have been mistaken. Tdap protects against all three- tetanus, diphtheria, and pertussis, as there currently isn’t a single known vaccine to only protect against pertussis.
Explanation: Diphtheria, tetanus and pertussis are prevented by a childhood vaccine, called DTaP, and a vaccine for adolescents and adults, called Tdap. A version for adults is also available without the pertussis component, called Td
