The FDA will postpone considering whether to extend emergency use authorization of Moderna’s (NSDQ:MRNA) COVID-19 vaccine over myocarditis concerns.

Some regulators have concluded that the risk of myocarditis is higher in recipients of the Moderna vaccine than the Pfizer-BioNTech vaccine.

Japan recently authorized men 30 and under who received an initial dose of the Moderna vaccine to receive a second dose of the Pfizer-BioNTech vaccine. Japanese authorities also recommended requiring a four-week interval between the first and second doses in this subpopulation — the same as the 28-day interval Moderna recommended for the second dose.

Sweden, Denmark, Norway and Finland recently recommended that younger age groups avoid obtaining the Moderna COVID-19 vaccine until further notice.

Health authorities in Europe are scrutinizing data to determine whether the Moderna vaccine increases the risk of myocarditis in younger recipients.

FDA data concerning the Moderna and Pfizer-BioNTech vaccines don’t reflect significant differences in myocarditis rates, Moderna Chief Medical Officer Paul Burton told WSJ.

Cases of myocarditis associated with mRNA vaccines tend to be minor and short in duration, although some vaccine recipients have been hospitalized over the condition.

In any event, the risk of contracting myocarditis is higher for those with COVID-19 infections than COVID-19 vaccine recipients.

The Moderna vaccine contains 100 micrograms of mRNA, which could increase short-term adverse effects while also providing a more durable immune response.

Moderna plans on recommending a 50 microgram dose level in its boosters.

In mid-day trading, MRNA shares were up 2.84% apiece to $333.41.