Trump threatened the same withdrawal in his first term, he is Trump so he can do whatever he likes. But one thing he always forgets is that;
it's only 4 years and will soon complete.
Imagine US withdraws their support and the government of these countries fails to provide these vaccines and drugs for their citizens,
Can the citizens afford the actual prices of these drugs?
Will the mortality rate not increase?
What will be the health care systems of citizens?
All your questions shouldn't be directed to the US citizens but to the world, it's the world that might suffer it, the US citizens will be just fine. Their pharmaceutical companies and research institutes are even steps ahead of the WHO, but they don't make announcements like the WHO that is saddled with the responsibility.
As much as I dislike this, Trump is right about one thing, as rich as China has become, what is it doing? China has always been far behind in funds allocations to world's projects.