They're using weakened viruses. Virus use in cancer is nothing new, but an interesting field. I can see why pharma hesitates using it, viruses can mutate, too, similar to cancer.

https://translational-medicine.biomedcentral.com/articles/10...

https://source.washu.edu/2021/03/zika-virus-helps-destroy-de...

But plant viruses, like cowpea mosaic, now that's a virus humans don't need to fear:

https://www.drugdiscoverynews.com/plant-virus-based-immunoth...