He was able to convince Trump early on that he could be a hero if he locked down to save lives and that they could get a vaccine in record time to eradicate Covid, which would also appeal to Trump's ego. Given how bad Tony botched the AIDS crisis and how he was never able to come up with a vaccine for HIV, this could also be his last shot at glory including a Nobel prize that eluded him and went to Montaigner.  Maybe he honestly believed they would prevent transmission without side effects of note?

This is just a theory but I can certainly envision how Trump with his ego could fall for the bait from Tony that he could be the savior if he went along. This is just one explanation. There are many others that combine political and financial incentives to explain what transpired. But many of the epidemiologists I respect were aware almost from day one that the lethality of Covid was confined to the frail and elderly with multiple comorbidities or the immune compromised.  That aspect has not changed at all but was drowned out by the media.

Big Pharma would be more than happy to go along! Why wouldn't they. You get subsidized, make billions, and are immunized from liability if things go wrong. And you can just say we were in an emergency pandemic and tried our best.

But the initial lie on the origin  and Tony's panic about what he had wrought with his behind the scenes financing of gain of function research set it all in motion. One lie leads to others as things never go according to plan!