The World Health Organization also said there was a "likely causal association" between coronavirus vaccines using mRNA technology and "very rare" heart inflammations.
Europe's drug regulator has found a possible link between very rare heart inflammation and Covid-19 vaccines from Pfizer-BioNTech and Moderna stressing that the benefits of the shots outweighed any risks.
The conditions, myocarditis and pericarditis, must be listed as side-effects of the two mRNA vaccines, the safety committee of the European Medicines Agency (EMA) said, adding that such cases primarily occurred within 14 days of inoculation.
It said cases occurred more often after the second dose and in younger adult men. This is in line with US findings last month.
Symptoms from the conditions include breathlessness, palpitations and chest pain.
EMA: Benefits of vaccine outweigh risks
EMA's safety panel also advised that people with a history of the rare blood disorder capillary leak syndrome (CLS), must not be vaccinated with Johnson & Johnson's shot.
While the regulator cautioned healthcare professionals and people to be on the lookout for post-vaccine symptoms, it said it was continuously monitoring the safety of approved vaccines as they are rolled out more widely.
"EMA confirms that the benefits of all authorised Covid-19 vaccines continue to outweigh their risks," it said.
EMA in total reviewed more than 300 cases of myocarditis and pericarditis in the European Union, Iceland, Norway and Liechtenstein - together referred to as the European Economic Area (EEA).
Most cases occurred with Pfizer's vaccine, called Comirnaty, the watchdog said.
The World Health Organization also said on Friday that there was a "likely causal association" between coronavirus vaccines using mRNA technology and "very rare" heart inflammations.
BREAKING: @WHO issues findings on mRNA vaccines & heart inflammation. "Very rare cases of myocarditis & pericarditis have been observed following vaccination with the mRNA #COVID19 vaccines. These cases occurred more often in younger men and after the second dose of the vaccine." pic.twitter.com/Bgs8SUZ4iT— Michael Bociurkiw (@WorldAffairsPro) July 9, 2021
The UN health body's Global Advisory Committee on Vaccine Safety (GACVS) said that cases of myocarditis – inflammation of the heart muscle – and pericarditis – inflammation of the lining around the heart – had been reported in multiple countries, especially the US.
"The reported cases have typically occurred within days of vaccination, more commonly among younger males and more often following the second dose of the Covid-19 mRNA vaccines," the committee said in a statement.
After reviewing available data, the GACVS judged that "current evidence suggests a likely causal association between myocarditis and the mRNA vaccines.”
Israel was the first country to report myocarditis among vaccine recipients in its fast-paced rollout of the mRNA shots.
Roughly 177 million doses of Comirnaty had been given in the EEA as of May 31, compared to 20 million doses of Moderna's Spikevax vaccine, EMA said.
The cases of myocarditis and pericarditis were "generally mild," and individuals "tend to recover within a short time following standard treatment and rest," Pfizer said in a statement.
EMA had also been looking into such cases with J&J and AstraZeneca vaccines, but on Friday said it had not found any causal relationship so far and had asked for more data from the drugmakers.
Both J&J and AstraZeneca vaccines use similar technology but with different versions of a cold virus to deliver immunity-building instructions to the body.
EMA in June asked CLS to be added as a side-effect of AstraZeneca's shot, Vaxzevria.
The regulator then too said people who had previously sustained the condition, in which fluids leak from the smallest blood vessels causing swelling and a drop in blood pressure, should not receive the shot.
For J&J, EMA reviewed three cases of CLS which occurred within two days of vaccination.
The developments could put pressure on vaccination plans in Europe and on the US company after millions of doses of the vaccine were banned for EU use over safety concerns following a contamination incident in the United States.
J&J did not immediately respond to a request for comment.