US regulators have expanded the use of Pfizer's Covid-19 vaccine to children as young as 12, offering a way to protect the nation's adolescents before they head back to school in the fall and paving the way for them to return to more normal activities.

Shots could begin as soon as Thursday, after a federal vaccine advisory committee issues recommendations for using the two-dose vaccine in 12 to 15-year-olds. An announcement is expected on Wednesday.

Most Covid-19 vaccines worldwide have been authorised for adults. Pfizer's vaccine is being used in multiple countries for teens as young as 16, and Canada recently became the first to expand use to 12 and up.

"This is a watershed moment in our ability to fight back the Covid-19 pandemic," Dr Bill Gruber, a Pfizer senior vice president who's also a pediatrician, told The Associated Press.