Transfer of vaccinia (smallpox vaccine) – a live and highly dermatotrophic virus – from the vaccination site to the skin and mucosa is a common adverse event of smallpox vaccination, due to a child's tendency to scratch the vaccination site that contains the live vaccinia virus. Accidental implantation may also occur from handling the gauze or other materials that cover or come in contact with the vaccination site.

Transfer can be by autoinoculation or by contact with a recent vaccinee. Accidental implantation may vary from a single lesion or multiple lesions to massive involvement of disruptive skin in children with eczema, acne, skin disorders with open lesions, and inflammatory eye diseases.

In individuals with cell-mediated immune defects, the implantation can be serious and life-threatening and lead to progressive vaccinia.

Children are most susceptible to more extensive inoculations because of their propensity to scratch the vaccination site.

The Advisory Committee on Immunization Practices (ACIP) advises that infants aged younger than 12 months not get the smallpox vaccine. In addition, the ACIP advises against nonemergency use of smallpox vaccine in children aged younger than 18 years.