3.5 stars. Very good and quite a page-turner, but there's something missing. Is it as good as The Stand (the book it's often being compared to)? Not even close. 
 Confusingly, the tone seems to shift throughout the book - it started like an exciting Michael Crichton-ish thriller but near the middle, I was plopped into some rubbish YA dystopian/zombie fiction (i.e The Forest of Hands and Teeth). Things improved in the last third which is why this book gets 3.5 instead of 3.