A woman receives the COVID-19 vaccine in Wales, UK. Matthew Horwood/Getty Breakthrough cases will become more common as the more transmissible Delta variant keeps spreading. But most disease experts still expect COVID-19 to be milder in vaccinated people. Two young people who got sick after they were vaccinated described how their symptoms progressed. See more stories on Insider's business page. Breakthrough infections, or COVID-19 cases diagnosed after someone is fully immunized, were supposed to be rare - at least that's what a report from the Centers for Disease Control and Prevention suggested in May.