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 eager 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 colleges. It aims to determine how well the Moderna vaccine prevents inoculated people from becoming unwitting carriers of the virus. To find an answer, researchers need to run an experiment — and make a big ask of college students: Flip a coin and get the vaccine now, or put off a shot for four months. Only by studying both groups will researchers determine how well vaccines work in stanching the virus’s spread.