no S - the people on life support machines who are dead...are dead.
There others on these machines who are not dead and they recover -mortality 15- 30% in most series.
There are others still who are in a permanent vegetative state (PVS)
which this young man was - from your ref:
"Leon Adams was 24 when he was left for dead by his attackers at a Cardiff train station. He spent two years in a coma and a further nine in a semi-vegetative state."
These are usually not on a ventilator, [but can be], and show sleep-wake cycles, some movement, breathe by themselves,
all of which the people who are dead on a ventilator ALL dont do, ever.
In my generation and possibly Sqad's Karen Quinlan's case caused widespread comment and a few television programs ('Are they really dead?' BBC1 springs to mind) 1990 I think
Nowadays it is Terri Schiavo and there are still acres of print about that, like this:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2676957/
Bryan Jennett summed it up nicely as: your relatives should say they think you are brain stem dead because then TWO sets of doctors are obliged to examine you and specify their results....