Vaccine dosages are based on age and not size or weight because they work differently than other medications. This is true for COVID-19 vaccines and other routinely recommended vaccines, like influenza or hepatitis vaccines. Medications are often based on weight because they are distributed through the bloodstream and work when a certain medication level is present. In this case, a person’s weight is important for the dosage. To achieve the same effects, a person weighing more will require more of a medication than a person weighing less. Vaccines work differently and are not distributed through the bloodstream. Vaccines imitate an infection when administered, activating the body’s immune response, which typically occurs at or near the injection site. Immune cells are directed to the injection site to attack foreign cells or proteins and produce antibodies that travel throughout the body and will protect against future infection. Because of how vaccines work, they typically require low quantities of active ingredients. Different dosages are evaluated during vaccine development to determine the lowest effective dose for the target age group. Clinical trials evaluate various dosing regimens to determine the best dosage and schedule that produces an adequate immune response which is both safe and effective. For more information, see Understanding How Vaccines Work.