Surely it'd be better overall if U.S. coronavirus case counts were declining everywhere. But one offset is that Phase Three testing of vaccines will be much faster and easier with the virus running relatively rampant than they would be with the virus scarce. For a randomized controlled trial to work, you need to vaccinate people and not vaccinate another group of people, then set both groups loose and see who gets infected or sick. If there's no virus around, no one in either group gets sick or infected. That's great for those people, but it doesn't much advance the vaccine toward the market, because there's no observable difference between the two randomly selected groups. If there is lots of virus around, then it's much less difficult to see whether the vaccine is helpful in preventing infection or sickness. This is a big deal, especially because there are significant ethical issues around the alternative approach of deliberately exposing vaccinated and non-vaccinated individuals to the virus.