The European Medicines Agency (EMA) said on Tuesday that benefits of the Johnson & Johnson vaccine outweigh risks despite finding a possible link to very rare cases of blood clots.
The EMA investigated a link between potentially deadly blood clots and the vaccine following eight cases in the United States, of which one was fatal.
On Tuesday, the EMA's safety committee said a warning should be added to the product information about the "very rare side effects" and health care professionals should inform their patients about the symptoms.
All of the reported cases occurred in combination with low levels of blood platelets. They happened mostly in women under 60 years within the first three weeks after the vaccination.
"This is a very rare effect, but it also makes it very important for doctors and patients to be aware of the sign so that they can spot any concerns and seek specialist help as soon as possible," EMA director Emer Cooke said at a press conference.
"Early intervention by specialists can change the outcome," she said.
Symptoms include blurred vision, headaches, shortness of breath and chest pain, for example.
US authorities suspended the use of the vaccine last week, after some 7 million people there had received the jab.
US company Johnson & Johnson halted the shipment of its jabs to countries in the European Union while awaiting clarification on the matter.
However, the EMA generally stood by its previous assessment, reiterating that the benefits of the jab in preventing Covid-19 outweighed the risk of side effects.
"The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of COVID-19 Vaccine Janssen in preventing COVID-19 outweigh the risks of side effects," the EMA said in a statement on Tuesday.
However, it had requested that the company conduct additional studies, Cooke said.
The chair of EMA's risk committee, Sabine Straus, said that her organization would continue monitoring possible side effects.
"We will analyze any new data, and any new evidence, as it becomes available, and provide updated guidance as needed," she said.
The announcement comes after similar reports for the vaccine produced by AstraZeneca. Several countries have suspended the use of the British-Swedish company's jabs for certain age groups.
However, it was too early to draw any conclusions on similarities between the two vaccines, Straus said.
European Commission President Ursula von der Leyen welcomed EMA's announcement. "This is good news for the roll-out of vaccination campaigns across the EU," she said on Twitter.
The European Union is in a race against time after a sluggish start of the vaccine roll-out across the bloc.