Why people who are fully vaccinated are getting breakthrough COVID infections

Even though COVID-19 vaccines are more than 90% effective at preventing serious illness, millions of vaccinated people will likely have a breakthrough infection, according to the Centers for Disease Control and Prevention. 

Most of the cases so far have been mild, with few or no symptoms. Out of more than 160 million fully vaccinated people in the U.S., the CDC says 5,500 — mostly the elderly and people with underlying health conditions — have been hospitalized or died.  Dr. Paul Duprex, a vaccine researcher at the University of Pittsburgh, says it's crucial people get vaccinated against the coronavirus so there's less virus to mutate and spread.