People receiving the Pfizer or Moderna Covid vaccines have been advised not to exercise after getting the jab.
The advice follows an "extremely rare" side effect of heart inflammation, which has been identified.
The two vaccines are among those being used in the UK on a daily basis and health experts have now identified an uncommon side effect that could emerge.
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Researchers found that both inflammation of the heart muscle, known as myocarditis, and inflammation of the heart's fluid-filled sac, known as pericarditis, have occurred particularly in men under 40 following their second dose.
It has led to people being advised to not exercise in the 48 hours following their second dose of the Pfizer jab, reports Wales Online.
However, the advice remains that the benefits of getting vaccinated far outweigh the risks in the majority of people.
The Medicines and Healthcare products Regulatory Agency (MHRA) and the UK Government's independent expert advisory body, the Commission on Human Medicines (CHM), have conducted a review into the potential harm of the vaccine.
The MHRA stated: "Healthcare professionals should be alert to the signs and symptoms of myocarditis and pericarditis.
"Vaccinated individuals should be advised to seek immediate medical attention should they experience new onset of chest pain, shortness of breath or symptoms of arrhythmia."
The experts state that these cases are typically mild, with individuals tending to recover within a short time following standard treatment and rest.
The European Medicines Agency (EMA), a medicines safety body, analysed cases across Europe and found:
- Pfizer: 145 cases of myocarditis and 138 cases of pericarditis out of 177 million doses given
- Moderna: 19 cases 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.
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.
There was no link found for vaccines such as Oxford-AstraZeneca or Janssen, which use a genetically modified virus.
