Europe's medical regulator has opened an investigation into possible links between Johnson & Johnson's (JNJ) COVID-19 vaccine and blood clotting events.
The European Medicines Agency (EMA) said in a statement on Friday it had started a "review" of the safety of the COVID-19 vaccine developed by J&J subsidiary Janssen, following reports that some patients who had received it had developed blood clots.
The EMA said it was aware of four instances of blood clots developing after the vaccine was administered, including one fatal case.
"It is currently not clear whether there is a causal association between vaccination with COVID-19 Vaccine Janssen and these conditions," the agency said in a statement. "PRAC [EMA's safety committee] is investigating these cases and will decide whether regulatory action may be necessary, which usually consists of an update to the product information.
"EMA will further communicate once the evaluation has concluded."
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Johnson & Johnson's shares were down 1% shortly after the statement, while rivals were mostly rising.
A spokesperson said it was aware of "a small number of very rare events following vaccination" and was continuing to work closely with regulators on safety issues.
"The safety and well-being of the people who use our products is our number one priority," a Johnson & Johnson spokesperson said.
"We continue to work closely with experts and regulators to assess the data and support the open communication of this information to healthcare professionals and the public to help ensure that, in the event of a very rare illness, appropriate steps can be taken for quick diagnosis and treatment."
The company said people who receive the vaccine should seek urgent medical assistance if they start experiencing symptoms such as shortness of breath, chest or abdominal pain, and swelling of the legs.
The Janssen vaccine is currently only being used in the United States under an emergency use authorisation. European regulators approved the vaccine for use across the EU last month but rollout has yet to commence.
The UK's medicines watchdog is currently assessing the Janssen vaccine for possible authorisation. Britain has already ordered 30 million doses.
Unlike other vaccines, Johnson & Johnson's formula requires only one injection for coverage. A study published in January concluded the vaccine was 66% effective at preventing COVID-19 and 85% effective at preventing serious cases of the virus.
News of the EMA's investigation into the Johnson & Johnson jab comes a day after the watchdog concluded there was a probable link between rare forms of blood clotting and AstraZeneca's COVID-19 vaccine. The watchdog said the side effect was "very rare" and benefits continued to outweigh risks.
Investigations suggested younger women were more at risk of developing the blood clotting complications after receiving the AstraZeneca vaccine, although it remains rare even in this group. UK authorities this week announced plans to avoid giving under-30s the AstraZeneca vaccine in the wake of the findings.