I really enjoyed this book, and I actually just finished reading. I finished it in a day because I really was interested in the outcome. My problem was 'suspension of disbelief." Yes, there are zombies. That alone requires a bit of 'suspension,' but accepting the existence of zombies is a given. The lab work and vaccine branch of the story was problematic for me. I just can't see any reason for using SPARTA. The researchers needed equipment and supplies from that company, not to mention the staff. I completely understand that. However, I do not believe the US military/government would make such a stupid decision.
There are several underground research facilities across the country. Even if they didn't go underground, they could have relocated to a sparsely populated area or moved to the west coast to buy time before the Zeds caught up. Everything at the SPARTA location, including staff and their families, could have been moved to a safe place. The people responsible for a desperately needed vaccine would not be left in such an exposed position. It doesn't make any sense. I had some difficulty with the ability to produce hundreds of thousands of vaccines so quickly too, but I could get past that. .