Epidemiology is on-topic at Medical Sciences Stack Exchange. The epidemiology tag there currently has 64 questions.
Epidemiology and Medical Science has overlap, but Epidemiology has aspects from sociology, political science, economics, and other things (we can see this by all the COVID-19 questions popping up everywhere across the Stack Exchange network, even though they don't necessarily fit in those places .. this site might give those places a more appropriate place to belong).
About medical science having 64 questions in the epidemiology tag, most of them are COVID-19 questions! They'd be in the Epidemiology SE if there was one, and the community over there has not done a good job of answering the epidemiology questions yet. Before COVID-19 and H1N1 there was hardly anything in that tag.
I also have my own personal reservations about the Medical Science SE. I tried to ask one question there and it got thrown in the trash mercilessly. I have doubts about it graduating past beta, they have 15 questions/day now but only because of COVID-19, it used to be one of the lowest (fewer than 1/day if I remember correctly). In 6 years they've managed to attract not many more users than Quantum Computing which is only 2 years. 68% answer rate is one of the lowest I've ever seen, probably because the mods there are quite strict.
Furthermore and foremost, just because it's on topic in one place doesn't make it a bad proposal. 100% of the Quantum Computing questions are on topic on either Physics, Chemistry, Mathematics, Computer Science, Theoretical Computer Science (which is usually on top on the previous two), Artificial Intelligence, MathOverflow, StackOverflow, Cybersecurity, and many others.
I would support an Epidemiology SE, but someone's going to have to ask some example questions or it will be closed due to not meeting the 3-day minimum requirements!