Trumps determination to hold on to his laurels for this may be his undoing, but it could be what amounts to a game of chicken. The moment he says maybe the vax has a net bad result for society is the moment the dems narrative will reverse.
If they reverse the narrative first, they may not be able to convince enough people that it wasn't them that unleashed this monster. If Trump gives them a crack to pry open, they'll say he knew all along, he caused this, he tricked us.
Trump’s honest response would be that he relied on the medical profession’s R&D and recommendations to treat Covid.