The US Food and Drug Administration on Tuesday authorised the use of a booster shot of Pfizer and BioNTech's Covid-19 vaccine for children aged 5 to 11, making everyone in the country over the age of 5 eligible for a third shot.
The US Centers for Disease Control and Prevention (CDC) still needs to sign off on the shots before they can be administered.
Children below the age of five are not yet eligible for a Covid-19 vaccine in the United States.
The US government has been pushing for eligible Americans to get boosters in the face of data that shows vaccine immunity wanes over time, and it recently authorized a second booster for people 50 and older. Roughly 102.3 million people have received a first booster dose.
The new authorization is meant to provide continued protection against Covid-19 for children aged 5 to 11, FDA Commissioner Robert Califf said in a statement.
It is unclear how many parents will opt to have their children in that age group receive a third dose. Just 28.8 per cent of children aged 5 to 11 are fully vaccinated, according to CDC data. That represents the lowest vaccination coverage out of all groups.
Paul Offit, a pediatric infectious diseases expert at Children's Hospital of Philadelphia, said booster shots in the age group would have a limited impact on the pandemic, even with evidence that the additional shots will protect against mild illness for three to six months.
"What is the goal of this vaccine? Are we trying to protect against all symptomatic infection for a limited period of time? Or are we trying to protect against serious illness, in which case all the evidence is that we are preventing serious illness," with the two-dose vaccine regimen, Offit said.
The CDC has scheduled a meeting of outside advisers to discuss vaccine boosters on Thursday. The agency's director has the final say on the administration of vaccines.