Can some one please explain to me why Bill Gates claims that vaccines will help in lowering the human population?

 Can some one please explain to me why Bill Gates claims that vaccines will help in lowering the human population??  How is something that supposedly meant to save lives actually lowers the population?? Below is the link of the clip where he says this. Watch the full video to understand it in context. He starts off by explaining the global warming problem and that it is created by a number of factors. One of these factors are the number of people on the plannet. Near the end of this clip, he claims that through things like reproductive health services and vaccines, the  population can be lowered. Now I understand why services involving reproduction can lower the amount of people but how do vaccines???? I guess Bill Gates is just another crazy conspiracy theorist. Maybe there is no good reason at all why this guy has huge shares in vaccines. Also no deal that he is getting so involved in vaccinating people in Africa