I've seen No Safety in Numbers by Dayna Lorentz described as The Hunger Games in a Mall, which I don't think is very accurate. No one is being entertained by what is going on. I've slaso seen it described as "apocalyptic." Nope. The best description I've come across is from the publisher. "A suspenseful survival story and modern day Lord of the Flies set in a mall that looks like yours."
A number of high school students, the same borderline cliche types you might see in a book with a high school setting, happen to go to the mall on the same day, at the same time. Unfortunately, it's a day and a time when a biological weapon is activated there. One of those kinds of biological weapons we hear about that causes people to get sick fast. The place is locked down. Use your imagination.
This is a good mash-up of traditional YA novel and adult thriller. There is no reason why this situation screams for YA characters. They could just as easily be adults. However, these teenagers really are teenagers, not adult characters passing as teenagers as I sometimes see in YA and adult thriller crossovers. Meaning this really is a YA book.
Though this is a first in a trilogy, I didn't feel I was being led on and teased with a nonending. The book was satisfying. Also, the book is from 2012. The rest of the series has been published, so you can binge.
A good example of why you should keep your eye out for older books you missed when they were shiny and new.