The 3-year-old sister of a newborn baby develops a cough diagnosed as pertussis by nasopharyngeal culture. The mother gives a history of having been immunized as a child. Which of the following is a correct statement regarding this clinical situation? a. The mother has no risk of acquiring the disease because she was immunized. b. Hyperimmune globulin is effective in protecting the infant. c. The risk to the infant depends on the immune status of the mother. d. Erythromycin should be administered to the infant. e. The 3-year-old sister should be immediately immunized with an additional dose of pertussis vaccine.