The problem: the second vaccine required live neural tissue from a volunteer who had never been infected. Harvesting it would kill the donor.
The series' defining (and controversial) hook is the method of delivery: the vaccine can only be administered to infected individuals through (specifically sexual intercourse). By doing so, Yu can return zombies to their human forms. This turns the typical survival horror narrative into a "panic horror" comedy where the protagonist must navigate a dangerous world while using his body as the literal "last hope" for humanity. Key Characters orenowakuchindakegazombieshitasekaiwosukueru