A mysterious post-apocalyptic plot that grabbed my attention right away

This book didn't waste any time jumping into events that grabbed and held my attention. I'm already wondering how the characters in the first pages of the book (a baby born following a car crash in the snow and a mysterious girl vandalizing a road sign) will connect to the rest of the story. The first chapters of the book introduce characters that are compelling (Kit and Nico) and give just enough detail from their backstories to make you want to know more about how they got to this state of the world where mysterious and dangerous flies abound. There is a very important dog character, Harry, which I'm always here for, though I'm already anxious to know whether Harry makes it through the book safely!