There are currently 80 active centres following participants in Canada, Poland, Australia, Hungary, China, Ukraine, and the Netherlands. Following randomization to one of the two treatment groups, participants will require clinic or phone follow-up visits every 3 months for 2.5 or 3.5 years. For participants on warfarin, INRs will be measured monthly or more frequently if required. For participants who stop warfarin therapy prematurely, every attempt will be made to have them restart it