All the COVID-19 vaccines have undergone a rigorous review process before being authorized for a given age group. The FDA’s evaluation of vaccines for young kids has been part of this overall thorough review process. Clinical trials were not started in children until after the trials in adults showed safety and efficacy of the vaccines.
Additionally, part of what made the review process longer for young kids is that experts were determining what dosage and series would be safe and effective for children under five. After reviewing initial data on the effectiveness of the vaccine in young kids, the FDA waited to receive additional findings from clinical trials to ensure that its recommendation was based on a substantial amount of clinical data.