The incident that led to the temporary closure and evacuation of the area, and to the deployment of a bomb disposal team took place on Wednesday, at a plant run by the pharmaceutical company Wockhardt UK in Wrexham, North Wales.
The staff was allowed back into the facility after the package was examined by a bomb squad and its content taken for analysis.
The local news website Wrexham.com reported then that a controlled explosion had taken place without damage to the area.
The man, named by the Kent Police as 53-year-old Anthony Collins, was arrested in Chatham the following morning and is scheduled to attend a local court on Saturday.
According to the police, the item sent to the Wrexham vaccine plant "was not a viable device."