Harvard interviews ~800 kids a year. The class size is 165, so if you assume that ~30 people turn down Harvard each year, approximately ~20% of kids who are interviewed are going to be offered acceptances. Last year, an HMS student told me that the kids whom the adcom liked more were offered interviews *later* than the kids whom they liked less, but I don't know if that's true this year. But it's possible that your chances may be >20% or <20%, depending on *when* you're invited to interview. In any case, ~20% seems to be slightly less than the accepted:interviewed ratio at the average school, so although you have a shot, you still need to absolutely nail the interview.