I remembered that I spoke to admission people from Harvard about their rate in Epi PhD at 2007 APHA, they said they would take 1 out of 4.
I guess it also depends on field, for example in epi, infectious disease is always more competitive than chronic.
applied for Epi and no news yet!! guess this year Epi gonna be really competitive, at least at Columbia, so many MPH applied to the PhD program, more than ever.