Lots of people in the medical/scientific community expected this result. The best explanation I have heard is that the vaccine companies planned these sub-optimal trials with 2-3 weeks between shots, because it meant getting the vaccines to market much more quickly. Instead of having vaccines available in December, they might not have been available until February.

So far, the evidence is pretty compelling that the vaccines have continued efficacy 6-7 months post-administration. It's obviously impossible to know much beyond a couple months however.