If an adolescent or adult who has never received a dose of Tdap is either infected with or exposed to pertussis, is vaccination with Tdap still necessary, and if so when?

Yes. Adolescents or adults who have a history of pertussis disease generally should receive Tdap according to the routine recommendation. This practice is recommended because the duration of protection induced by pertussis disease is unknown (waning might begin as early as 7 years after infection) and because diagnosis of pertussis can be difficult to confirm. Administering pertussis vaccine to people with a history of pertussis presents no theoretical risk. For details, visit CDC’s published recommendations on this topic at www.cdc.gov/mmwr/volumes/67/rr/pdfs/rr6702a1-H.pdf.