How does Justin Cronin portray mortality through the character Amy in The Passage?

In The Passage, Amy is one of the people who are forced to become test subjects for the virus that turns humans into vampires. She presents hope for the survival of the human species. Amy is only six years old when Brad Wolgast, a federal agent, kidnaps her and her guardian, a nun named Lacey. The vampires escape the research facility where the experiments were conducted, which sets off a military conflict which very few humans survive, but Amy goes into hiding with Wolgast, who has recanted his position.
In the subsequent developments, a century passes before Amy resurfaces in the post-apocalyptic United States, which is largely ruled by the vampires. She has only aged a few years and is still a child. It turns out that the version of the virus that infected her is more advanced, and she is not a vampire (now called a viral). Several human (non-vampire) survivors aid her in finding and rejoining Sister Lacey, who likewise has barely aged. In the ensuing battles and nuclear bombing, many of the vampires (known as virals) are killed, and the humans, along with Amy, escape to form a new colony.
Although the novel ends ambiguously, it seems likely that Amy holds the key to immunity or possibly immortality. Although she is pure and uncorrupted, her supporters are mortal and must die, leaving it to the future generations to discover whether her biological resistance was an anomaly or can be adapted to save other humans.