A toddler and a 4-week-old infant have just been diagnosed with pertussis. What should be done to prevent transmission to the infant?

• A. Administer the initial diphtheria, pertussis, and tetanus vaccine.
• B. Instruct the parent to limit contact between the toddler and the infant.
• C. Order azithromycin 10 mg/kg/day in a single dose daily for 5 days.
• D. Prescribe erythromycin 10 mg/kg/dose four times daily for 14 days.

Answer: (C)

Post-exposure prophylaxis with a macrolide antibiotic for all close contacts is the way to prevent pertussis transmission to a vulnerable infant. Immunization won’t provide immediate protection, and asking parents to limit contact isn’t reliably protective in the household. The best option is to give azithromycin to the exposed contacts in a dosing regimen that’s easy to administer and well tolerated in young children—10 mg/kg once daily for 5 days. This short, simple course effectively reduces nasopharyngeal carriage and lowers the chance of spreading pertussis to the infant. Erythromycin can be used, but it requires a longer, more burdensome course with more GI side effects, which is why azithromycin is preferred for prophylaxis in this setting.