I think the opening sequence would have worked better if they had used the alternate ending (which I could debate the pros and cons of). The alternate ending had the trio hiding in the hospital that these experiments were taking place in soon after the Brendan Gleeson character got infected. Now whether or not the alternate ending would have been better is debatable. Less cliche' than the whole "Humans are the real monsters" idea, but wasn't really anything special either. And I really enjoy the climax of the film as is.