"An earlier version of this article incorrectly described actions taken by regulators in Sweden and Denmark. They have halted use of the Moderna vaccine in children; they have not begun offering single doses. The article also misstated the number of Covid hospitalizations in U.S. children. It is more than 63,000 from August 2020 to October 2021, not 900,000 since the beginning of the pandemic. In addition, the article misstated the timing of an F.D.A. meeting on authorization of the Pfizer-BioNTech vaccine for children. It is later this month, not next week," the lengthy correction stated in full.