"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.
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.
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.