'I don't think this is a serious issue:' State health experts weigh in about 'vaccine breakthrough' cases Why ‘breakthrough cases’ are possible after COVID-19 vaccine Health officials explain how the vaccine is effective, but there are reports of .01 percent of fully vaccinated Washingtonians testing positive for the virus. SEATTLE - Washington state health leaders answered questions regarding "vaccine breakthrough" cases of COVID-19 on Wednesday. It’s the term health experts are using for anyone testing positive for COVID-19 more than two weeks after being fully vaccinated. The state has identified 102 breakthrough cases so far between Feb 1 to March 20. It raises questions of demographics, variants, and the vaccines administered.