Accidental implantation can be by autoinoculation or contact inoculation. The total number of accidental implantations observed during a large U.S. study in 1968 were as follows:

As indicated from these data, accidental implantation is one of the most common adverse events following primary vaccination. It is far less common after revaccination but the threat of transfer to contacts remains.

Although no age group is spared, infants and children are most susceptible to more extensive inoculations because of their tendency to scratch an itching vaccination site. Older individuals may be able to control such scratching, despite the itching, but younger individuals most often cannot. Older individuals tend to have fewer lesions but they are seen more frequently. Older individuals implant virus frequently on the face as a result of inadvertent contamination of the hands or via fomites. Minute injuries, such as occur in shaving establish the potential for implantation.

Individuals who are particularly susceptible to accidental implantation, either by autoinoculation or contact inoculation include those with:

These individuals are also more susceptible to serious disease as a consequence.

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