The World Health Organization said Friday that there was a "likely causal association" between mRNA COVID-19 vaccines and "very rare" heart inflammations, but that the benefits of the jabs still outweighed the risks as they reduce both hospitalizations and deaths.

In a statement, the WHO said that reports, especially from the United States, of two rare conditions, myocarditis, an inflammation of the heart, and of its lining, called pericarditis, had typically occurred within days of vaccination, mainly among younger males after the second dose.

"Very rare cases of myocarditis and pericarditis have been observed following vaccination with the mRNA COVID-19 vaccines," it said, referring to the two vaccines using such technology, by Pfizer-BioNTech and Moderna.

"The benefits of mRNA COVID-19 vaccines outweigh the risks in reducing hospitalizations and deaths due to COVID-19 infections," it said.

The WHO judged that "current evidence suggests a likely causal association between myocarditis and the mRNA vaccines."

Available data suggested myocarditis and pericarditis following vaccination was "generally mild" and responded to treatment such as rest or non-steroidal anti-inflammatory drugs, the WHO said. "Follow-up is ongoing to determine long-term outcomes," it said.

"Vaccinated individuals should be instructed to seek immediate medical attention if they develop symptoms indicative of myocarditis or pericarditis such as new onset and persisting chest pain, shortness of breath, or palpitations following vaccination," it added.

In June pediatricians across the U.S. had reported several cases of temporary heart inflammation in teenagers after they received the Pfizer/BioNTech COVID-19 vaccine.

The previously healthy adolescents – all boys – developed chest pain within four days after their second dose. MRI exams showed myocarditis, or heart muscle inflammation.

Earlier on Friday, Europe's drug regulator said it had found a possible link between very rare heart inflammation and COVID-19 vaccines from Pfizer-BioNTech and Moderna. It too stressed that the benefits of the shots outweighed any risks.