Moderna says Covid vaccine has fewer breakthrough cases than Pfizer's, but higher myocarditis rates in young men

Moderna defended the use of its Covid-19 vaccine Thursday, saying the protection it offers against severe disease, hospitalization and death outweighs the risk of myocarditis, a rare heart condition seen in a small number of young men who received the shot.

The company announced last week that the Food and Drug Administration needed more time to decide whether to authorize its two-dose vaccine for use in children ages 12 to 17 as the agency looks into reports of myocarditis, or the inflammation of the heart muscle.

Reported cases of the rare heart inflammation in men under age 30 are relatively higher after Moderna’s vaccine compared with those who received the shots made by Pfizer and BioNTech, Moderna Chief Medical Officer Dr. Paul Burton told reporters on a call Thursday.

Burton cited data from France on males ages 12 to 29. It showed there were 13.3 cases of myocarditis per 100,000 people for Moderna’s vaccine compared with 2.7 cases per 100,000 people for the Pfizer vaccine.

However, he also touted data from the Centers for Disease Control and Prevention that showed the rates of mild or severe disease from Covid were lower in Moderna recipients than in those who received Pfizer’s or Johnson & Johnson’s vaccines. For example, there were 86 breakthrough cases per 100,000 people for the Moderna vaccine. That compares to 135 breakthrough cases per 100,000 for Pfizer’s, he said.

The data also showed unvaccinated people have an elevenfold increased risk of dying compared with those who have gotten the shots.

“While I think health authorities are carefully assessing the data, being appropriately cautious, you can see that they continue to recommend the use of the mRNA-1273 Moderna vaccine,” he said on the call. “We believe that the balance of benefit and risk is extremely positive,” he added.

More than 71 million Americans are fully vaccinated with the Moderna vaccine, according to data compiled by the CDC.

Europe approved Moderna’s vaccine for use in adolescents in July. However, some countries, such as Finland and Sweden, have since paused the vaccine’s use in people 30 and younger due to concerns about myocarditis.

Scientists are still trying to figure out why young men experience the heart condition after vaccination, but some hypothesize hormones may play a role, specifically testosterone, as well as the fact that Moderna’s vaccine uses a higher dosage of mRNA than Pfizer’s, Burton said.

“I do think this hypothesis of testosterone is important,” Burton said. “We know that there is indeed some inflammation associated with testosterone. … We do have in the primary series, as you know, 100 micrograms of mRNA, so we have slightly higher levels of spike protein, and that could be a contributing factor as well.”

Burton said the company has not seen any cases of myocarditis in those who received its booster shot, which is half the dosage of its primary series of shots. He said the company will continue to monitor for cases. The reported cases of myocarditis are generally mild with symptoms usually resolving on their own, he said.