I would say that maybe we're assuming too much in terms of the global situation. It could very well be the case that there are other parts of the world unaffected that may yet come to play in this story (if at all).
All that we really know about the global situation is that the outbreak hit West coast USA, Japan, China and Hawaii. Maybe whatever happened to cause the outbreak has just hit areas around the Pacific ocean. Could give us a lot of
hope for the rest of the world to have survived, or at least put up a defence. Maybe the worlds survivors have taken to the seas in fleets of ships and humanity becomes a seafaring, migrant race, who knows. Either way there's still hope yet.