Sweden and Denmark said they will pause the use of Moderna's Covid-19 vaccine for younger age groups after reports of possible rare side effects, such as myocarditis.

The Swedish health agency said it would pause using the shot for people born in 1991 and later as data pointed to an increase of myocarditis and pericarditis among youths and young adults that had been vaccinated. Those conditions involve inflammation of the heart or its lining.

"The connection is especially clear when it comes to...