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Moderna stock fell more than 6% in morning trading as US approval on Moderna's Covid-19 vaccine for teens delayed.

US biotech firm Moderna said Sunday (Oct 31) that American officials have delayed approving its Covid-19 vaccine for teenagers to allow more time to better assess the potential risk of developing myocarditis, or heart inflammation.

The US Food and Drug Administration (FDA) on Friday "informed Moderna that the agency requires additional time to evaluate recent international analyses of the risk of myocarditis after vaccination," the biotech company said Sunday in a statement.

The evaluation on whether to recommend Moderna's vaccine for 12- to 17-year-olds could last until January 2022, the company said.

Myocarditis and an associated risk, pericarditis (inflammation around the heart) have previously been linked to the Pfizer and Moderna Covid-19 vaccines themselves, particularly among adolescent boys and young men.

But the risk is far higher after infection with Covid-19, according to a study published in August by the US Centers for Disease Control and Prevention (CDC).

The CDC found that "myocarditis following vaccination with mRNA vaccines has been rare and generally mild," Moderna said.

Another mRNA Covid-19 vaccine, by Pfizer-BioNTech, on Friday received US authorization for children aged five to 11, in smaller doses than for adults.

Moderna had previously said it was waiting for the FDA to decide on whether to approve its vaccine for adolescents under age 18 before seeking regulator approval for younger age groups.