It really is hard to say but with numbers like that I'd have to say pretty bad like an entire batch of whiskey was bad and a few hundred drank it at the very least, it really is a total shot in the dark though as it may not kill you the first shot or even 10, also some people may have died from it and not had this caught as C.O.D., or injured and not reported it. What I can say is that it is pretty bad to have killed 15 people and is probably because of it being spiked. It can also cause serious G.I. tract problems like liver and kidney damage as well as death and blindness. Remember these are just the REPORTED from this incident.
Given that people were blinded it is likely contaminated with a large % of methanol or consisted only of methanol.
The breakdown products of ethanol compete with the toxic breakdown products of methanol so slight contamination isn't a problem. (brandy) I believe the cure for anti-freeze and methanol poisoning is ethanol.
If the wrong ingredients were used methanol could be produced. Perhaps they typically "cut" their booze with methanol and just used a greater % than the ethanol could inhibit.