The cover stars are Madison Headrick and HRH Princess Sirivannavari. (source)
Click here to view last year cover featuring Karlie Kloss, here to view 2017 over featuring Kate Upton, here to view 2016 cover featuring Behati Prinsloo, here to view 2015 cover featuring Praya Lundberg, here to view 2014 cover featuring Iris Strubegger, here to view 2013 cover featuring Coco Rocha.