I don't see any reason to round down instead of up, but this is a 3.5-star book to me.
Some characters, locations and themes strongly echo the author's Newsflesh trilogy. The speculative science is interesting enough and did not stretch my suspension of disbelief too hard, although I found the emphasis on the hygiene hypothesis a little ... preachy, almost.
The big "reveal" at the end was predictable from chapter 1, so the book became an exercise in waiting for the main character to finally put 2 and 2 together. Nevertheless, it was still exciting enough to finish in one day.
