Once you understand a little computer science, you realize that there is no possible antivirus program that detects all threats. It is not even possible in principle. It is flatly ruled out by at least two fundamental features of computation.

Antivirus however is not useless. As in most areas of life, most virus writers aren’t very skilled or are very lazy. Thus, AV programs have some use.

But against a skilled attacker (like the NSA or the Mossad), antivirus programs are basically completely worthless, even if these intelligence agencies have no special backdoors in place.