Answer: When it is a medical device. Especially when it is an experimental medical device.
In this video clip (9m:17s) Dr David Marshall explains that the Pfizer and Moderna mRNA experimental medical products are actually a medical device. They are neither a vaccine nor a therapeutic agent. (See partial transcript below.)
It is a totally new device, where a piece of RNA is encapsulated inside a ball of fat and injected into the muscle of the recipient. The device then co-ops the body’s cell machinery to manufacture the spike protein of the coronavirus.