A few “super carriers” with off-the-charts viral loads are likely responsible for the bulk of COVID-19 transmissions, while about half of infected people aren’t contagious at all at the time of diagnosis, suggests a new CU Boulder analysis of more than 72,000 test samples. A second, related study lends further credence to the idea that viral load, or the amount of virus particles a person carries, drives contagion. It found that only one in five university students who tested positive while living in a residence hall infected their roommate. And their viral load was nearly seven times higher than those who didn’t spread the virus. “The takeaway from these studies is that most people with COVID don’t get other people sick, but a few people get a lot of people sick,” said Sara Sawyer, a professor of molecular, cellular and developmental biology and senior author of the first study. “If you don’t have a viral super-carrier sitting near you at dinner, you might be OK. But if you do, you’re out of luck. It’s a game of roulette so you have to continue to be careful.”