The Food and Drug Administration on Monday authorised the vaccine for children aged 12 to 15, offering relief to parents eager to get their children back to schools and summer camps.

The action by the CDC group is an important, but not required, final seal of federal regulatory approval.

President Joe Biden hailed Wednesday's vote, noting that means 17 million more people in the US now qualify to get vaccinated.

"I encourage their parents to make sure they get the shot," he said. "As I promised last week, we're ready. This new population is going to find the vaccine rollout fast and efficient."

The youngest age previously approved for the Pfizer vaccine was 16 years old.

Some states, including Georgia, Delaware and Arkansas, began offering the vaccine to young teens on Tuesday.

California's main COVID-19 website said families could start making appointments for the younger group on Thursday.

The Advisory Committee on Immunization Practices, which provides recommendations to the CDC, voted 14-0 to back the vaccine after reviewing trial evidence.

That showed no one in the 12-to-15 age group who received the vaccine got COVID-19. There were no severe allergic reactions.

Moreover, the vaccine produced robust antibody responses in the age group and showed 100 per cent efficacy in the trial, with no cases of symptomatic COVID-19 among the fully vaccinated adolescents.

About one-third of all Americans have been fully vaccinated, according to the CDC data. But the pace of vaccination has slowed in the recent weeks.

Children have been considered by health officials as being at a lower risk for severe COVID-19, but they can still spread the virus.

Pfizer is running a separate vaccine trial in children as young as six months old, and has said it expects data on two- to 11-year-olds in September.

Australian Associated Press