According to the article, they were originally trying biotech methods, but then shifted to straightforward selective breeding, which worked better.
For already existing viral strains, that's to be expected. I don't know if you've ever had discussions with synthetic biology students but... as Hugh Hixon at Alcor once said to me, "that is the stuff of nightmares, assuming you can even sleep afterwards." Fully-novel genetic constructs, hybridization of various unlike genomes, or even more potentially exotic constructs such as fungal spores that upon contact with human secretions re-express (medusa-like) into something akint o Toxoplasmosis gondii only inducing schizophrenia, hyperaggression, ...
I'm woefully underinformed on this topic, but this doesn't seem good at all:
I feel as though I ought provide more commentary instead of just an article dump, but I feel more strongly than that that what I have to say would be obvious or stupid or both, so.