A 12-year old girl receives the Pfizer-BioNtech vaccine in New York City on May 14th, 2021.Adam Jeffery | CNBCMore than half a million 12- to 15-year olds have received a Covid-19 vaccine so far — less than a week since the Centers for Disease Control and Prevention cleared it for public distribution, Director Dr. Rochelle Walensky said Tuesday.On Wednesday, the Advisory Committee on Immunization Practices endorsed the use of the Pfizer Covid-19 vaccine for adolescents aged 12 to 15. Children make up 20% of the U.S. population and Pfizer previously said that studies showed its vaccine is 100% effective in kids aged 12 to 15.”Last week FDA authorized and CDC recommended use of the Pfizer vaccine for adolescents. In less than one week, we have vaccinated more than 600,000 12- to 15-year-olds, and in total, more than 4.1 million adolescents aged 12 to 17 have been vaccinated so far. My own son was one of them,” Walensky said.President Biden has a goal of vaccinating 70% of adult Americans by July 4, vaccinating adolescents will only bring us closer to the 70% to 85% that experts say is needed to achieve herd immunity.Hospital admissions and deaths related to Covid-19 are down in the U.S., and the country is administering 1.4 million shots per day, according to the CDC. The U.S. is seeing its lowest daily case numbers since June 2020.This story is breaking. Check back here for updates.Correction: This article was updated to correct Biden’s July 4 goal. He wants 70% of all adult Americans to get vaccinated by that day.