A closer look at breakthrough COVID-19 cases in vaccinated residents in Houston-area Low percentage of vaccinated people contracting virus HOUSTON – In this latest wave of the COVID-19 pandemic, breakthrough cases in people who have been fully vaccinated are getting a lot of attention, but doctors say they are extremely rare. “They tend to be seen in folks who for whatever reason can’t mount a great immune response, like people who are elderly, immunocompromised, folks on dialysis folks on chemotherapy,” said Dr. Linda Yancey, infectious disease expert at Memorial Hermann. Local health departments say they identify breakthrough cases by cross-checking positive lab results with Texas Health Trace the state’s COVID-19 disease reporting tool and ImmTrac, the state’s immunization database.