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SAN DIEGO — The County of San Diego on Tuesday reported its first known COVID-19-associated death of a person who had been fully vaccinated against the virus.

The woman, who was in her 70s, had a pre-existing medical condition and was hospitalized before she died, a county official told FOX 5 in an email.

The woman, who was not publicly identified, was one of 458 fully-vaccinated individuals who have become infected with COVID-19 this year, and one of four fully-vaccinated people to become hospitalized, according to the county.

The small percentage of fully vaccinated individuals who still get COVID-19 are referred to as vaccine breakthrough cases and are expected, according to the U.S. Centers for Disease Control and Prevention. As of June 1, the CDC received reports of 535 people with breakthrough infection who died. Sixteen percent of these cases were reported as asymptomatic or not related to the virus.

In February, the county reported its first known case of a fully vaccinated individual to test positive for the virus.