Europe's drug regulator has found a possible link between very rare heart inflammation and Covid-19 vaccines from Pfizer-BioNTech and Moderna, it said, 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'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.
Roughly 177 million doses of Comirnaty had been given in the EEA as of 31 May, 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 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.