Moderna announced Tuesday it is beginning a study of its COVID-19 vaccine in children younger than 12 – one that will include babies as young as 6 months.

The announcement comes exactly a year after the first adult received a test dose of the shot, created at the National Institutes of Health. It’s now being used across the U.S. and in multiple other countries.

Moderna also has tested the vaccine in 12- to 17-year-olds but hasn’t yet released the findings. The study in younger children will be more complex, because researcher need to determine whether to us smaller doses than in adults and adolescents.

The study aims to eventually enroll about 6,750 children in the U.S. and Canada. That’s after a phase-in portion to determine the best dose to test in children age 2 and older and the right dose in those younger than 2.

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