WASHINGTON (AFP) - The US Food and Drug Administration (FDA) on Thursday (Dec 9) expanded authorisation of the Pfizer-BioNTech Covid-19 booster to include 16- and 17-year-olds, amid growing concern over the Omicron variant's ability to overcome immunity conferred by two shots.

The FDA's decision comes a day after Pfizer and BioNTech released the results of laboratory tests that showed three doses of their vaccine appeared effective against Omicron, while two doses were probably not enough to stop infection - though may still prevent severe disease.

Even before Omicron emerged, there had been concerns about waning immunity over time, and the United States and many other northern hemisphere countries are experiencing winter waves of infections driven by the currently dominant Delta strain.

The expanded emergency use authorisation, which applies six months after the primary vaccination series, is expected to be quickly reviewed and ratified by a second agency, the Centres for Disease Control and Prevention.

"Vaccination and getting a booster when eligible, along with other preventive measures like masking and avoiding large crowds and poorly ventilated spaces, remain our most effective methods for fighting Covid-19," said acting FDA chief Janet Woodcock.

In a statement, the FDA said the evidence supporting the decision came from previously collected immune response data from people aged 18 and older, which they inferred could be extended to 16- and 17-year-olds.

Rare cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the outer lining of the heart) have been linked to the use of mRNA vaccines in younger males.

Both the Pfizer-BioNTech and Moderna shots use mRNA technology, with Moderna packing a higher dose.

But the FDA said the accumulation of real-world data, together with the increasing number of Covid-19 cases, showed that the benefits of a booster in this age group outweighed the risks.