The U.S. health regulator is delaying its decision on authorizing Moderna Inc's COVID-19 vaccine for adolescents to check if the shot could increase the risk of heart inflammation, the Wall Street Journal reported on Friday, citing people familiar with the matter.

Moderna's shares fell nearly 2% to $326.25 in midday trading after the report.

The U.S. Food and Drug Administration (FDA) has been looking into the risk of myocarditis among younger men vaccinated with Moderna's shot, especially compared with Pfizer's vaccine, after certain Nordic countries limited use of the shot, the Wall Street Journal report said.

The agency has not yet determined whether there is heightened risk, the report added.

Earlier this month, Finland, Sweden and Denmark paused the use of Moderna's COVID-19 shot for younger males due to reports of the side effect, myocarditis, in men.

The Danish Health Agency, however, later said the vaccine was available to under-18s, and that a statement on Wednesday suggesting a suspension had, in fact, been a miscommunication.

The FDA and Moderna did not immediately respond to requests for comment.

(Reporting by Amruta Khandekar; Editing by Devika Syamnath)