"half full"
I'm sorry to disappoint, but i'm pretty sure that 'half full' are those with confirmed appointments, i.e. those that said they would go. there are still some 400 or so outstanding offers of appointment that may or may not go- so if 80% of those accept their appointment, then really close to 80% of the class is full. today those figures should change, even this week is likely to have a lot of replies because last weekend was one with lots of ivy league accept/reject notices and it might impact appointment decisions.
Don't lose hope though, I'm still waiting too!