I cannot talk about Harvard specifically (where I was rejected) but it is possible to get an interview in a top 5 program with boards in the 220s. You just have to do an away and impress the hell out of them. I emailed a PD from a top 10 program after I was rejected and he told me that, given the extremely high quality of the applicant pool, Step 1 scores were a fast and easy cut-off (249 average at the school in question). Caveats occur if you do an away rotation there and impress.
However, I believe Ursus is correct in that there are a lot of other things that enter into the equation. For example, if you are not vastly superior in one area to compensate for average boards (e.g. Junior AoA, several publications w/ PhD, very, very strong LORs from world-famous attendings) then you probably won't have a shot.