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Heart inflammation is a "very rare" side-effect of the Covid vaccines made by Pfizer and Moderna, according to regulators in Europe.
The European Medicines Agency said the side-effects were more common in younger men.
The medicines safety body said the benefits of Covid vaccines continue to far outweigh any risks.
But doctors and patients have been advised to be aware of the symptoms of heart inflammation.
These include chest pain, a feeling of breathlessness and a pounding or fluttering heartbeat. Anyone with these symptoms should see a doctor.
Two conditions were linked to the vaccines – inflammation of the heart muscle itself, known as myocarditis, and inflammation of the fluid-filled sac the heart sits in, known as pericarditis.
The EMA analysis of cases found:
- Pfizer-BioNTech – 145 cases of myocarditis and 138 cases of pericarditis out of 177m doses given
- Moderna – 19 case of myocarditis and 19 cases of pericarditis out of 20 million doses given
Five people died. The review said they were all either elderly or had other health conditions.
The UK's Medicines and Healthcare products Regulatory Agency (MHRA) has also been investigating the link.
It reported: "A consistent pattern of cases occurring more frequently in young males and shortly after the second dose of the vaccines.
"These reports are extremely rare, and the events are typically mild with individuals usually recovering within a short time with standard treatment and rest," it added.
Most cases are thought to be within 14 days of vaccination.
While the risk is very rare, it is more likely to develop in young people – who are currently the focus on the vaccination campaign in the UK.
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Concerns about the side-effects have already played into the UK debate around vaccinating children, who are at lower risk of Covid.
Myocarditis and pericarditis will be officially listed as side-effects in the UK and Europe, mirroring a move by the regulators in the US last month.
"The chance of these conditions occurring is very low, but you should be aware of the symptoms so that you can get prompt medical treatment to help recovery and avoid complications," the EMA said.
The link with heart inflammation was found only in the vaccines that rely on mRNA technology to train the immune system.
The was no link found for vaccines such as Oxford-AstraZeneca or Janssen, which use a genetically modified virus.
However, the EMA has advised anyone with a history of capillary leak syndrome should not be given the Janssen vaccine. This is a rare but serious syndrome in which fluid leaks from blood vessels in the body.
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