The 99% detection rates they boast sound great until you consider the countless thousands of old viruses that are catalogued and the sampling bias inherent in only testing viruses they know about. The new and custom viruses may represent a small fraction of that catalogue but that doesn't mean they aren't more widespread. And there really is no good way to measure performance on zero day viruses. If had reason to suspect my PC was compromised I'd format and reinstall.
And don't think these viruses slipping through are super viruses engineered by the CIA or mob. I remember custom virus kits being sold so you could have an undetectable virus for less than $1000 which you can then use to infect hundreds of PCs and maybe thousands before your virus is found and catalogued.
Antivirus is a bandaid solution to bad security models.
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