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 US 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.