Norway and Sweden are advising men younger than age 30 and all people under 18 years old to avoid taking the Moderna COVID-19 vaccine because of its rare association with myocarditis and endocarditis.

Analyses of reported adverse reactions from the U.S. “have suggested that myocarditis may be more frequent when using Moderna’s vaccine as a second dose” than using the Pfizer-BioNTech vaccine, “but the numbers have been small and therefore uncertain,” said Geir Bukholm, deputy director general at the Norwegian Institute of Public Health.

“New monitoring data from Ontario, Canada, substantiates that this observation is correct, and preliminary monitoring data from Norway, Sweden and other countries could indicate the same,” he said.

The varying reporting systems make it impossible to accurately determine the incidence of carditis, but “the side effect is rare and the absolute risk is still low,” he said.

Both of the mRNA vaccines used in the Norwegian coronavirus immunization program are “good and very effective, and provide a high degree of protection against severe COVID-19 infection,” although the Moderna vaccine does seem to provide marginally better protection against the Delta variant than the Pfizer vaccine, Bukholm said.

Nordic countries are collaborating on several large health registry analyses to detect and investigate rare COVID-19 vaccine side effects. While the combined data aren’t complete, Norwegian data suggest an increased incidence of cardiac inflammation after using the Moderna vaccine as a second dose.

Both governments already recommend the Pfizer vaccine for people aged 12-17 years. The new recommendation is that those under age 18 who received the Moderna first should have the Pfizer vaccine as their second dose. — Michele G. Sullivan