The answer is given in the first 60 seconds of this documentary when Gates says the investment return on vaccines is 20 to 1. But the story does not end there. Beyond incredible profits there is the lure of power over the entire human race, even to using vaccines delivered by mosquitoes to reduce population. This is the madman who is shaping the future of humanity. 
2020 Dec 28 - Source: Truth Comes to Light - https://open.lbry.com/@Truth_Comes_to_Light:6/Bill-Gates---The-deleted-documentary---Copy:5