So according to In The Pipeline[1] they essentially modified their existing MERS vaccine so that it worked for SARS-COV-2. Their original vaccine had already gone through phase 1 trials and had shown efficacy and no harmful side effects.
Derek Lowe is terrific in communicating complex ideas in the field to concerned lay observers like me; he's been a great resource for reliable information about the science of this thing.
Just to clarify; the ChAdOx1 MERS prospect hasn't proceeded to a trial demonstrating efficacy in human subjects (as yet).
That's a particularly important distinction here, given the nature of the novel delivery mechanism[1] and prior observation of complications in SARS-CoV-1 vaccine development[2]
[1] https://blogs.sciencemag.org/pipeline/archives/2020/04/23/a-...