From a biology standpoint, adolescents, defined as those between age 12-17, are very similar to adults. The FDA also provides regulatory guidance for clinical trials and includes 12-17 year olds together because of these similarities. Once we move into children younger than 12 years of age, we begin to see more variability in weight, pubertal status, and other factors that might influence responses to drugs and vaccines.

Children are not small adults. They may have either different side effects, requirements for different dosing, or different immune responses than adults. Therefore, most vaccine clinical development strategies include testing an experimental vaccine in young, healthy adults, before considering them in either older—potentially more frail—or younger individuals.

The initial clinical trials were conducted in the age groups where we saw the greatest risk of severe COVID-19 disease and virus transmission, to maximize immediate impact of the approved vaccines and streamline the management of these large trials. The trials for younger individuals were being planned and set up while the adult trials were ongoing, giving us the best ability to test the vaccines as efficiently as possible.

Vaccines and other investigational products are always evaluated first in adults, because children are considered to be a special and more vulnerable group. Safety must be established in adults first, and then the vaccine can be tested in younger and younger age groups, with safety checks along the way. The type of trial plan that does this is called ‘age de-escalating,’ and the major COVID vaccine manufacturers are conducting this type of trial program. In addition, dosage often has to be adjusted in younger age groups, so the current trials are also ‘dose-finding’ trials.