From what the article says I think the original virus works by attacking tumors which then explode and spread the virus all around the body. If they use a version of the virus that is safe or that the person is immune to then it would target the cancer and cause it to explode but afterwards be harmless.
That's just the impression I get from the article. I know literally nothing about this do if anyone with actual knowledge can explain properly please do!
(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?
That's just the impression I get from the article. I know literally nothing about this do if anyone with actual knowledge can explain properly please do!