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.
The theory goes that the body’s immune system then responds to make making antibodies against the disease. But effectively the technique is giving the recipient a form of the disease that is endogenously made. (It is not surprising that many of the trial recipients got China virus symptoms after their first shot.)
He said it doesn’t even come close to the definition of a vaccine. A vaccine is meant to 1) rev up the immune system thus producing immunity to the disease, and, 2) interrupt transmission of the disease.
This is done by introducing to the immune system a weakened version of the virus so that the immune system makes antibodies to that pathogen. But in the case of the mRNA vaccines the body may not even recognise the wild virus when it sees it, or it may actually assist the wild virus to enter the cells in a process called antibody enhancement, or pathogenic priming. That makes the disease much worse, even deadly.
But by classifying the experimental agent as a vaccine the manufacturers get immunity from prosecution if things go wrong and they have on this mRNA one.
But even more so, they can then slip other devices in there such as injectables and call them a vaccine. This is what Bill Gates is planning with his ID tracking chip.
Partial Transcript
This is not a vaccine.
We need to be really clear. We’re using the term “vaccine” to sneak this thing under public health exemptions.
This is not a vaccine. This is an mRNA packaged in a fat envelope, that is delivered to a cell.
It is a medical device designed to stimulate the human cell into becoming a pathogen creator.
It is not a vaccine. Vaccines actually are a legally defined term, and they’re a legally defined term under public health law, they’re legally defined term under the CDC and FDA standards.
And a vaccine specifically has to stimulate both an immunity within the person who is receiving it, but it also has to disrupt transmission.
And that is not what this is. They have been abundantly clear in saying that the mRNA strand that is going into the cell, it is not to stop transmission. It is a treatment.
But if it was discussed as a treatment, it would not get the sympathetic ear of the public health authorities, because then people would say, well what other treatments are there?