Erythema Infectiosum (Fifth Disease, Parvovirus B19)

A 7-year-old immunocompetent girl presents to the pediatric clinic with a 5-day history of rash. On examination, she has a bright erythematous confluent macular rash on both cheeks with characteristic circumoral pallor (the "slapped-cheek" appearance marked **C** in the diagram), followed by a lacy, reticular erythematous rash on her trunk and extremities that waxes and wanes with temperature changes. She is afebrile and well-appearing. Two classmates have a similar rash. Which of the following best explains why this child is NO LONGER INFECTIOUS despite the active rash?