Articles often claim that the Delta variant is more virulent, e.g. “Citing the spread of the more virulent Delta coronavirus variant in the United Kingdom”. Earlier in the year the same was said about the Alpha (Kent) variant, that it was more “virulent”.
That was untrue. Virulent means “harmful”, not “infectious”.
If anything, the evidence suggests that the Delta variant may be less virulent, but more transmissible/infectious—although it is hard to be sure this is true, given that the vulnerable old are now protected by vaccines.