(I also know nothing about this stuff.) From the article I gathered that, if the person is immune to this anti-cancer virus, it won't have any chance to do its job before it gets eradicated from the system. Which seems to be a problem: after first attempt this treatment won't work again since one's immune system will definitely recognize the virus (having immune system weakened might do the trick though).

The article also mentions that most people in US are vaccinated—I'm not sure how to read that: vaccinated from this particular virus?