They basic premise that I can surmise is that they were ahead and most aggressive early with their vaccine program. As such, they were also going to report results earliest in the world.
Therefore, the efficacy of the first two doses seems to wane. Potentially as soon as six months. They will then be more susceptible again, and are are now facing the issue of Delta and other variants. A booster also appears to be in their future
Hence, great immediate results but their successes are slowing degrading over time as new mutations hit the horizon.