Dozens of Vaccinated Residents Have Died of COVID in Mass.: What to Know About Breakthrough Cases Seventy-nine vaccinated residents in Massachusetts died from COVID-19, either without being hospitalized or following a hospital stay Nearly 80 people have died with COVID-19 in Massachusetts even though they were fully vaccinated as the more contagious Delta variant continues to spread. These breakthrough cases where fully vaccinated individuals test positive for coronavirus have so far been rare, but are possible. Here’s what you need to know about breakthrough cases in Massachusetts: How many breakthrough COVID cases are there in Massachusetts? Public health officials have tracked 4,450 “breakthrough” cases of COVID-19 among Massachusetts residents fully vaccinated against the disease, representing about one-tenth of 1% of the roughly 4.2 million people immunized.