STANFORD (CBS SF) — Stanford medical researchers began administering Pfizer COVID-19 shots Wednesday to children ages 2-to-5 years old as part of a nationwide trial to determine the right dosage for the age group.

While phase 1 is already fully enrolled with volunteers, parents can still sign up their children age 5-12 for a larger trial at Stanford later this summer.

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Dr. Yvonne Maldonado, a pediatric infectious diseases expert at Stanford Medicine, is leading the trial.

“We need to make sure that children are protected,” Dr. Maldonado told the San Francisco Chronicle. . “There’s been over 3 million children infected, thousands hospitalized and almost 300 deaths. Even though it’s not the half a million-plus adult deaths we’ve seen, we’ve seen an impact in children.”

A total of 144 participants have been signed up across the country for phase 1. Researchers will be injecting three different doses of the vaccine to determine safety and tolerability.

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“We want to make sure they’re protected,” Dr. Maldonado said. “And they can transmit to others so we want to make sure they can protect the community as well.”

According to the American Academy of Pediatrics, nearly 3.5 million children have been infected with COVID-19, representing about 13.5% of overall U.S. cases.

Of those, 347 children have died from the virus.

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Pfizer is currently awaiting FDA approval for widespread distribution of their vaccine to children ages 12 to 15 years old.