BOULDER, Colo. – A week before she would have become eligible for a coronavirus vaccine, Madden Brewster agreed to forgo a shot for months to help scientists answer one of the most pressing questions of the pandemic: When can we all stop wearing our masks? Brewster, a 24-year-old graduate student at the University of Colorado at Boulder, is as impatient as anyone for the pandemic to end, but delaying her shot may be her most powerful way to contribute. She is one of the first volunteers in a massive study unfolding at 20 college campuses. It aims to determine how well the Moderna vaccine prevents vaccinated people from becoming unwitting carriers of the virus.