Some countries are limiting teens to 1 dose of COVID-19 vaccine, citing very rare heart complications which can follow a 2nd shot

• Norway, Hong Kong, and the UK moved to give young people only a single dose of COVID-19 vaccine.
• Data suggests that a type of heart inflammation is slightly more common in adolescent males.
• Authorities in the US - where more children are badly ill - decided full vaccination outweighs the risk.

Some countries are offering only one dose of COVID-19 vaccine to children, citing caution over the very small risk of health complications in adolescents.

Officials from Norway, the UK, and Hong Kong recommended children be offered only one dose, citing instances of heart inflammation after a second dose.

Real-world data has suggested that adolescents, especially young men, are at a slightly higher risk of heart complications, which are very rare and are mostly mild.

Estimates vary, but according to CDC data published in June, the rate of myocarditis in males 12-29 is about 0.004%, Insider previously reported.

Authorities in the US have decided to continue with full vaccination in younger age groups. The New York Times reported that higher rates of severe COVID-19 in US children compared to other nations gave officials a different risk-benefit calculation.

After reviewing evidence, Centers for Disease Control and Prevention advisors in June calculated that even if all US boys aged 12 to 17 were fully vaccinated, only around 70 would get myocarditis - the heart-inflammation symptom.

The same calculus found that those shots would prevent 5,700 infections, 215 hospitalizations, and two deaths, the Times reported.

Although the conditions are usually mild, it is not clear if the condition may be permanent, Jeremy Brown, an expert in respiratory diseases at University College London and a member of Britain's vaccine advisory group, told The New York Times.

"That makes it very hard for us to make the absolute statement that it's totally safe to give this vaccine," Brown told The Times.

In the UK, where high rates of adult vaccination have kept children mostly out of the hospital, "the chance of getting severe COVID in a healthy 12- to 15-year-old is almost negligible," Dr. Brown said.

"Against that, you have to make sure that the vaccine that you're giving is utterly safe."

On Wednesday, Sweden said it was suspending the use of the Moderna vaccine because of a study from Denmark's Statens Serum Institut which suggests that myocarditis and pericarditis, an inflammation of the heart or its lining, were more common in children and young adults.

Officials again said that those conditions remain very rare, according to a statement from the Sweden public health agency.

The results have not yet been shared as the study is expected to be published at the end of the month.

The heart complication usually goes away on its own, but it is still a good idea to have it checked by a doctor, per the Sweden public health agency.

Denmark also suspended the use of the Moderna vaccine based on Sweden's findings, and Finland on Thursday also suspended Moderna, but only in teenage boys.

Two studies published on Wednesday in the New England Journal of Medicine also support that young males are at higher risk of the very rare heart complications after vaccination.

A study from Israel found that males aged 16 to 29 had the highest risk, with about 11 in 100,000 males getting a new heart condition after getting vaccinated. Most of the cases included in the study were described as mild or moderate.

The second study, also from Israel, found that boys 16 to 19 years old were at highest risk from myocarditis after the second dose of vaccine. Although very low, the risk was around nine times higher than in unvaccinated boys, per The New York Times.