This is actually a very optimistic manga if it posits 80% of humanity can die off from a virus without the survivors going full Mad Max/Last of Us dystopia eleven times over
So much whooooshing over the head in this thread. Go back and read the first chapter side by side with the last chapter. Notice what's the same and what isn't.
If you still need the mangaka to spell out T H I R D T I M E L I N E for you after that, big yikes