By AMY DiPIERRO and PAT EATON-ROBB, Associated Press STORRS, Connecticut (AP) — At most of the largest U.S. public universities, students are under no obligation to be vaccinated against COVID-19. Some schools do require vaccines, but with leniency for those who opt out. Still others have expelled students who do not comply. As a new semester begins amid a resurgence of the coronavirus, administrators