Trump didn't create the vaccine and doesn't understand it. That isn't a knock. It's just the truth. He trusted people because he was trying to do the right thing and protect the nation from what he'd been led to believe was an extremely serious threat. He has a personal Achilles' heel of being a self-admitted "germaphobe" and was probably easily steered on covid, at least in the beginning.

In short, although he's great, he's still a person, and not infallible. He believed the hype when he shouldn't have, and he trusted people who aren't to be trusted. He now at least openly admits that he shouldn't have trusted these people.

Furthermore, the vaccine is experimental and is only approved for "emergency use." It is not something that should be distributed widely to every man, woman, and child right now, if ever.