I imagine very few seats are filled because people tend to make decisions later in the Spring. What matters is how many interview slots are left.
If the Wash U MSTP statistics are representative of the nation at large (and that is not a given, but it is the data I have)
http://mstp.wustl.edu/admissions/Pages/Statistics.aspx
then about 80% of interview slots have been filled by this point and there are about 2-3 times as many applicants under consideration as there are interview slots.
Again, MSTP cycle is not really like MD cycle and the two could not be comparable. However, if I remember from a UChicago presentation this summer, by mid September there are usually more applicants than there are interview slots and the disparity only grows from there.