. >>> during spring break college freshman chelsea siefert received her first dose of the moderna vaccine. when she returned to the university of chicago she ate with her usual group of friends. a few days later she started feeling ill and tested positive for covid-19. >> the reason there's an outbreak on campus is because of spring brace. >> reporter: chelsea is isolating. >> i would say most people have gotten their first shot. >> it takes about two weeks for any immunity to begin to present itself. >> reporter: dr. florez tells his vaccinated patients to continue masking and staying apart when not at home. florez says even with both doses any exposure could still pose a danger. >> we have seen some reports of what are called breakthrough infections were someone has been fully immunized and they still get infections. what we suspect is there may be a potential for someone fully vaccinated to carry the virus to someone who hasn't been. >> the vaccine helps protect against severe illness. cases like this gives them a chance to understand how to lengthen immunity and protect against varian