Scientists speculate that while this change slows down reproduction and decreases virulence it also makes it harder for the immune system to track down virus in

Scientists speculate that while this change slows down reproduction and decreases virulence it also makes it harder for the immune system to track down virus infected cells, so it basically slows down the infection. In the long run zoonotic viruses tend to become less deadly. But I wouldn't expect this sort of mutation to be selected for when the number of cases is increasing exponentially.