“The Swedish Public Health Agency has decided to suspend the use of Moderna's vaccine Spikevax, for everyone born in 1991 and later, for precautionary reasons. The cause is signals of an increased risk of side effects such as inflammation of the heart muscle,” the Swedish Public Health Agency said.

At the same time, the Danish Health Authority said that people under the age of 18 would not be offered the Moderna vaccine out of precaution.

“In the preliminary data, which is collected from 4 Nordic countries, there is a suspicion of an increased risk of heart inflammation, when vaccinated with Moderna, although the number of cases of heart inflammation remains very low,” it said.

The Moderna COVID‑19 vaccine, codenamed mRNA-1273 and sold under the brand name Spikevax, is a COVID-19 vaccine developed by American company Moderna, the United States National Institute of Allergy and Infectious Diseases (NIAID), and the Biomedical Advanced Research and Development Authority (BARDA).

It is authorized for use in people aged twelve years and older in some jurisdictions and for people eighteen years and older in other jurisdictions to provide protection against COVID-19 which is caused by infection by the SARS-CoV-2 virus.

It is designed to be administered as two or three 0.5 mL doses given by intramuscular injection at an interval of at least 28 days apart.

On June 23, the US Centers for Disease Control and Prevention (CDC) confirmed that myocarditis or pericarditis occurs in about 13 of every 1 million young people, mostly male and over the age of 16, who received the Moderna or the Pfizer–BioNTech vaccine.