How soon the vaccines are available to children will depend on how long the clinical trials and the FDA review process take. In the most optimistic scenario, a vaccine could be available for large numbers of kids, especially older ones, in time for the start of the school year in the fall. But last week, Operation Warp Speed said that Moderna was having trouble recruiting enough participants in its trial for adolescents, having enrolled only 800 out of a planned 3,000. The company’s CEO also said that while Moderna would soon begin trials for kids ages 1 to 11, it did not expect to have results until 2022. Pfizer’s adolescent trial has reportedly finished enrollment, though the company declined to specify when it plans to move on to younger children.

The FDA fast-tracked the COVID-19 vaccines for adults using a process called emergency use authorization. But the normal vaccine-approval process may be more appropriate for children, says Vanderbilt’s Tina Hartert, who is leading a study on the incidence of COVID-19 in kids. Approval will take more time for several reasons. For example, the FDA has said that it wants to see more safety data—six months or more depending on the novelty of the vaccine technology—compared with the two months required for emergency use.