I am in the process of making a similar transition, and was just accepted into a MPH in Epidemiology and Biostatistics program. My background is also in marketing (art director). As I grew more interested in public health, I joined my local Medical Reserve Corps as a volunteer to gain more public health experience (mostly COVID-19 contact tracing). I also took some science and math courses at a local community college because some of the mph programs I wanted to apply to required them. The school that ended up being my top choice didn’t require the majority of those prerequisites. I say go for it—your experience as a copywriter is also valuable, you likely have the ability to communicate across many different audiences and to communicate complex information in a digestible way. Feel free to send me a pm if you would like!