The Centers for Disease Control and Prevention on Thursday signed off on booster shots of the Pfizer-BioNTech Covid-19 vaccine for children ages 5 to 11 at least five months after their second doses. The final endorsement, from CDC Director Rochelle Walensky, comes as Covid cases are surging yet again in the U.S., fueled by the omicron subvariant and an offshoot of that strain.