Chief Medical Officer Paul Burton acknowledged on the call that the company’s COVID-19 vaccine does appear linked to increased chances of an inflammatory heart condition known as myocarditis in young men. Moderna maintains that the benefits of the vaccine continue to outweigh the extremely rare risk of myocarditis, he said Thursday. 

Sweden and Denmark have halted Moderna shot’s use in younger people as they monitor the risk of heart inflammation, while Germany has favored the vaccine from Pfizer and BioNTech over Moderna’s in younger and pregnant people, citing potential side effects. Moderna has fallen behind those main rivals in clearances for booster shots and use in children. 

Compared with the Pfizer-BioNTech vaccine, Moderna’s shot has led to only 10 additional cases of myocarditis per 100,000 inoculations among males age 12 to 29, Burton said. The occurrence of the side effect in males suggests that the hormone testosterone may be important, he said. The chief medical officer took only a handful of questions before the call concluded. 

As health officials have cited concerns about the vaccine’s potential heart risk, Moderna’s market value has taken a blow. Shares of the Cambridge, Massachusetts-based biotech have plummeted 34% this month through Wednesday’s close. They gained as much as 0.9% as of 9:32 a.m. in New York. 

The U.S. Food and Drug Administration has required additional time to assess Moderna’s emergency request to approve the vaccine in adolescents aged 12 to 17 as it examines the rare risk of myocarditis. The regulatory review may not be completed until January. 

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