as jake says alot of these jabs have been carried out without this happening, so it would appear to be rare. yes shocking, but lets not over sensationalise.
from the experiences of the other girls in the school, it could be possible that they had a bad batch of the drug. an investigation is under way, but until then no point in speculating on the mass media outrage and national campaigns (no doubt lead by the Daily Mail) that may ensue.
lets hope this is an isolated tragedy.