Code reviews are not meant to prevent bugs. They are for making sure an individual's effort is good enough for the team to assume further maintenance of that piece of code. That almost never means that the code must be bug-free, it means that other people than the author can follow the thought processes expressed in the code. If that leads to other people finding shallow bugs, more power to them. But that doesn't mean that bug prevention is (or should be) the primary goal of code reviews.

First and foremost, code reviews are for enforcing team standards, such as testing coverage, documentation, and adhering to scope. "Our code reviews aren't preventing bugs" is not a useful metric in that sense. If you're relying on code reviews to prevent bugs, you have other problems in your development process. If I spot a bug during code review, my first question would be "why wasn't this bug found by the automated test suite?"