Postdoc to transition from theoretical field (Statistics) to applied field
Answering perspective: I'm a PI with a degree in Epidemiology currently actively looking for postdocs.


  Is it possible to transition from (bio)statistics to epidemiology during a post doc and then be competitive for an academic job afterwards?


Absolutely. This would be a good way to get applied experience that would definitely help you reframe yourself as a methodologically sophisticated epidemiologist, and there are a number of departments that you could then fit in - either combined Epi &amp; Biostatistics Departments, or Epidemiology departments that heavily value methods. 

Alternately, there are also a number of smaller "groups" which have biostatisticians in academic jobs where their primary tasks are working on applied problems. These often appear in medical schools or research institutes, where there is a need for consistent, good, but not necessarily groundbreaking biostatistical support. Some of these might also be staff scientist positions, but I've also encountered some with biostatisticians in faculty positions (albeit ones that are largely soft money funded). For those, having a postdoc in an applied area where you can show that you've done work in a setting with real data (and real data's problems), and can produce results that are interpretable in an applied field is potentially a considerable boon.

I know a number of people who have gone this route, moving from a more theoretical discipline (either statistics or applied math) towards epidemiology, and their skillsets are often viewed as quite valuable.