Good for Serbia for implementing some centralized system. A PCR test result in the U.S has absolutely no way of being verified, either negative or positive In Israel, which swabs every person who lands at the airport. There is a suspiciously steady stream of people flying in from the U.S who were negative before take off and positive 10 minutes after landing
The problem is at the state level, you have a lot of weird political stuff that influences what and how each state performs certain processes.
There’s also a huge flow of travel from US to Israel, and lots of weird corner cases and loopholes. (Just like Canada and Mexico)