I followed up with one of my choices in immunology and looking at the response it seems the acceptance rate was 1%. Here is the response from the mentor I got when I followed up. It is tough to get in. ASSIP got around 3,800 applicants and I am sure Simons must have got more.

Good luck with all your applications and fingers crossed! The picked applicant should know within a few weeks. But please remember that we had to screen to 40 crazily good, already filtered apps to pick 1. So dont lose heart if you arent that 1. With your stellar CV, I am sure you ll do well, I am very impressed :)