I'm not sure this is entirely correct. Ron Hoskins "hygienic" bees, which operate in a slightly different mode (chewing the phoretic mites) have had some initial research done on them and it was found that they had a non-lethal form of DWV that protected them from the lethal form. More research on viruses seems to me to be the way forward but Ratnieks seems very quiet on the subject of DWV viruses. Maybe he's embarrassed that he did not think of this line of study.
Wouldn't it be ironic if we found that he hasn't been breeding hygienic bees but instead he's bred mites and bees that carry Type b DWV, just like Ron Hoskins'!