"Likely for children that young [it] will be a three-dose vaccine which means it likely won't be for a couple of months at least before it's approved," Fauci said during an interview with CNN on Wednesday.
Fauci said vaccine tests on two- to four-year-old children has not met the criteria for approval by the authorities.
Children between the ages 5 and 12 are approved only for so-called "primary" coronavirus vaccine shots, made up of two Pfizer doses, and many have only begun their inoculation in recent weeks.
Centers for Disease Control and Prevention Director Rochelle Walensky said in earlier remarks that US health authorities hope to make a decision in the coming days or weeks on coronavirus "booster" vaccines for children 12 to 15 years of age.