The risk of myocarditis and other heart muscle diseases more than doubles in a year after a covid-19 infection, according to the report.

The measured risk of blood clots in the lungs increases by 74 percent, but since it is an unusual diagnosis among children, it is a matter of a few cases - and thus the uncertainty in the measurement becomes large.

Taking the statistical margin of error into account, it is a minimum of 12 percent and a maximum of 172 percent.

For kidney failure, type 2 diabetes and disturbances in the heart rhythm (arrhythmias), the risk increase is calculated to be 19-30 percent - or 9-54 percent if you include the margin of error.

Unusual diagnoses

The diagnoses are uncommon and affect only a few children and young people each year.

The most common of the mentioned diagnoses were disturbances in the heart rhythm, which affected 0.5 percent of the children who had been infected and 0.4 percent of those who had not been ill.

Blood clots in the lungs were rarest and affected 13 children per 100,000 infected and 8 per 100,000 who were not infected.

The figures apply to children and young people aged 0-17 who did not have any of the 24 studied symptoms before the investigation period.

Pediatricians: Worrying long-term effects

Stuart Berger, president of the American Academy of Pediatrics AAP's Section of Cardiologists and Cardiac Surgeons, says the report shows the long-term effects of covid-19 are real, worrisome and potentially very serious.

- The message we should take with us is that we should be very keen to use all methods to prevent covid, especially the vaccine, says Berger to the American health news site WebMD.

The report is based on a comparison between roughly 781,000 children and young people who had laboratory-confirmed covid-19 and over 2.3 million who did not.

The data is taken from a health insurance database operated by the company HealthVerity and does not include all children and adolescents of the current age.

Thus, it is not certain that the participants are representative of the entire age group, write the report authors.