Well, ideally you would know more than just the number of patients. The roughest estimate would be to compute the expected variance of a binomial distribution with N=120e3 and P=0.12, which gives a standard deviation of 0.00094, so a difference of 0.5% is well over 5 standard deviations out. Not quite good enough for particle physics, but you're damn right it's good enough for medicine.

Now you just have to look at group sizes, possible confounding factors, etc...