Probably not since January, but maybe after the phase 1 trials and the dosing schedule was figured out. To be honest, if it doesn't come out that they were using it off the books, either they're suckers or we are.
https://arstechnica.com/science/2020/11/astrazenecas-best-co...
But that's a complete, uneducated (but I think, logical) guess.
Or it doesn't come out because they are good at keeping it secret.
Considering high-level officials in the government apparently never received the vaccine, I would be surprised if the people involved in the research took the vaccine.
Then again, maybe secret was kept extremely well. My intuition is, if POTUS knew about it, he would have been telegraphing such on twitter back in the summer months.