Like many common vaccines, the mRNA COVID-19 vaccines should have been a three-dose series. The first two injections prime the immune system, and the third boosts it to provide a higher antibody response. In 5 to 11 year olds, the booster more than doubles the antibody levels found following the second dose. Additionally, boosting has been shown in other age groups to further improve the antibodies so that they better bind the virus and provide more protection against variants.