Indian National Immunization Schedule (UIP)

A 2-week-old neonate born at a government hospital in rural India received routine immunization at birth as per the UIP schedule. The vaccine marked **A** in the diagram was administered by intradermal injection over the left deltoid. Two weeks later, the mother notices a small ulceration at the injection site with mild local swelling. She is concerned this represents an infection or adverse reaction. Which of the following best explains this clinical finding?