US drugmaker Moderna has confirmed that the Food and Drug Administration (FDA) has delayed granting the emergency use authorisation (EUA) to its Covid vaccine for teenagers aged 12 to 17, over the risk of myocarditis — inflammation of the heart muscle.

The FDA needs more time to complete its assessment and the review may not be completed until January 2022, the drugmaker said in a statement late on Sunday.

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The “FDA has notified the Company that it will require additional time to complete its assessment of Moderna’s Emergency Use Authorisation request for the use of the Moderna Covid-19 vaccine (mRNA-1273) at the 100 Aug dose level in adolescents 12 to 17 years of age”, the statement said.

On Friday evening, the FDA informed Moderna that “the agency requires additional time to evaluate recent international analyses of the risk of myocarditis after vaccination”, it added.

Moderna said that the company is fully committed to working closely with the FDA, and will also conduct its own “careful review of new external analyses”.

Moderna also said it will delay filing a request for emergency use authorisation for a smaller dose of the vaccine for younger kids aged six to 11 while the FDA completes its review.

Earlier in May, the drugmaker said that its Covid vaccine was 100 percent effective in a study of 12-to-17-year-olds. The company had then applied to expand the emergency use of its vaccine for adolescents in June.

An increased risk of myocarditis has been described for Covid-19 vaccines, including the Moderna Covid-19 vaccine, particularly in young men and following the second dose. The US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have stated that myocarditis following vaccination with mRNA vaccines has been rare and generally mild.

It is estimated that over 1.5 million adolescents have received the Moderna Covid-19 vaccine.

The observed rate of myocarditis reports in those less than 18 years of age in Moderna’s global safety database does not suggest an increased risk of myocarditis in this population.