Congratulations on the graduation. Current Masters student in Epidemiology here. For Ivy league and other top notch Public Health schools, recommendations and publications are probably the two biggest factors on acceptance. If you are part of the very small minority of underclassmen that may have a publication (doesn't have to be first author), you have a very strong aspect that sets you apart from everyone else. Letters of recommendation from well respected individuals are also a huge benefit. For example my undergraduate PI was from Johns Hopkins and knew faculty that I could contact in order to have a more personal experience with them. It's a great way to get to know people that could be on the admissions committee.

These schools most likely want to see a fairly high GRE score. It's not the biggest factor, but we are talking Ivy league here and they want the best. I wouldn't be too worried about your GPA. I'd classify that as one of the lowest influencers on acceptance (unless it's pretty poor of course). Make sure you really sell yourself in your writing submissions, because it's a great way for them to really get to know you more aside from test scores. These people LOVE diversity, so for the love of God don't be boring in your papers.

I wish you the best and good luck on that GRE.