A school-age child has died from a rare complication of measles contracted in infancy, Los Angeles County health officials said Thursday.

The child, who had been too young to be vaccinated when they were infected by the virus, died of subacute sclerosing panencephalitis, according to the county health department. The incurable disorder causes progressive brain damage and is nearly universally fatal.

About 1 in 10,000 people who get measles develops the disorder, but the risk is 1 in 600 for infants.

“This case is a painful reminder of how dangerous measles can be, especially for our most vulnerable community members,” said Dr. Muntu Davis, Los Angeles County health officer. “Infants too young to be vaccinated rely on all of us to help protect them through community immunity.”

This has been the worst year for measles in the U.S. in more than three decades, as childhood vaccination rates decline and domestic and international outbreaks have spread. The Centers for Disease Control and Prevention confirmed 1,454 cases as of Tuesday. Three people have died.

The measles, mumps and rubella vaccine is safe and is 97% effective at preventing measles after two doses. Doctors recommend kids get a shot at 12-15 months old and a second one at age 4-6 years.

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