The World Health Organization (WHO) and regulators in Europe have found a potential link between mRNA vaccines and heart inflammation conditions such as myocarditis - inflammation of the heart muscle - and pericarditis - inflammation of the sac-like structure surrounding the heart.

The European Medicines Agency (EMA) said that the side-effects occur in "very rare cases" but are more common in men and boys under 30 and shortly after the second dose of the vaccine.

Symptoms include feeling breathless, irregular heartbeat and chest pain.

The analysis by the European medicines safety body found that out of 117 million doses of Pfizer administered within the EU, Iceland, Norway and Liechtenstein there were 145 cases of myocarditis and 138 cases of pericarditis. In the case of another mRNA vaccine, Moderna, there were 19 cases of myocarditis and 19 cases of pericarditis out of 20 million doses given.

The WHO says that while the conditions can lead to serious illness, "they are often mild and respond well to conservative treatment."

But in five cases people who developed these heart conditions died. The EMA said they were of advanced age or had concomitant health conditions.

The conclusions of the European regulators confirmed the U.S. findings last month, when the Centers for Disease Control and Prevention concluded that heart inflammation after a second dose of Pfizer was higher than expected among young men.

The EMA didn't find the same potential links between heart inflammation and other COVID-19 vaccines in either AstraZeneca or Johnson & Johnson. But they advise that people with a history of the rare blood disorder capillary leak syndrome should not be given the Johnson & Johnson shot, while already in June they asked for capillary leak syndrome to be added as a potential side-effect of AstraZeneca.

Both the EMA and the WHO concluded that the benefits of the COVID-19 vaccines still outweigh the risks, but EU regulators advised that these side-effects should be formally listed in the product information for the mRNA vaccines as warnings for doctors and patients.

In other news surrounding the Pfizer vaccine, Israel reported on Monday a drop in efficacy of Pfizer in preventing infection as cases of the Delta various increase across the nation, but said that the vaccine remains efficient in preventing serious illness.

Pfizer has also said that it is working on creating a booster jab that could be used after the two doses to increase efficacy in the case of highly contagious variants such as Delta. EU regulators say that it's still too early to determine whether more than two shots are needed.