A 6-month-old infant from rural Maharashtra presents with a 2-week history of paroxysmal cough with inspiratory stridor and post-tussive vomiting. The mother reports that an older sibling had similar symptoms 3 weeks ago. On examination, the infant is afebrile with mild respiratory distress. A nasopharyngeal swab for PCR is positive for Bordetella pertussis. The infant has received only one dose of pentavalent vaccine. What is the most appropriate immediate management?