The European Medicines Agency says there is no link between the use of the Oxford University-AstraZeneca vaccine and the development of blood clots – but scientists are still exploring possible links.

A number of European countries temporarily banned the vaccine following reports of blood clots in people who have received the jab.

The European Medicines Agency (EMA), the World Health Organization and AstraZeneca all insist the vaccine is safe and say the numbers of such incidents are what would be expected in the general population if no one had been vaccinated.

But scientists in Europe are now trying to find out whether the vaccine could be responsible for a rare type of blood clot in the brain that has been reported in several people who had the jab.

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During a press briefing on Thursday, EMA health experts said the vaccine was not associated with an increased risk of thromboembolic events or blood clotting. 

But the EMA hasn't yet "definitively" ruled out an association with rare cases of blood clots in vessels draining the blood from the brain, known as cerebral venous sinus thrombosis (CVST).

Most of the rare blood clots have been seen in women and most cases have been reported in Europe. Two cases have been reported in India.

European investigators have also put forward one theory that the vaccine triggers an unusual antibody in some rare cases. Others are trying to understand whether the cases are linked with birth control pills.

But many scientists say there is no definitive evidence and it is not clear whether or why AstraZeneca's vaccine would cause an issue not shared by other vaccines that target a similar part of the coronavirus.

Experts in Germany and Norway, where several cases of blood clots have been reported, are exploring a hypothesis that the vaccine could be triggering an immune response in which the body produces antibodies that could result in blood clots.

Paal Andre Holme of Norway's Oslo University Hospital, which treated three healthcare workers with severe blood clots after they received the AstraZeneca vaccine, told a news conference on Thursday that "we've made discoveries" that could "explain the clinical progression of our patients."

Holme warned that the findings were preliminary. "This is only the beginning of all the research that is being done," he said. He did not release any data supporting his hypothesis.

A team of German researchers at Greifswald University Clinic on Friday said they came to a similar conclusion. If proven correct, there may be a way to treat the condition, the scientists said.

The EMA says that there have been fewer than 30 cases identified out of the 20 million doses administered in the UK and the European Economic Area. 

The regulator recommended a warning label be added and a public campaign launched to raise awareness of the signs and symptoms to help mitigate any possible side effects. 

June Raine, the head of the UK's medicines and healthcare products regulatory agency, said that although cases of CVST were extremely rare and no link to the vaccine had been proven, "as a precautionary measure we would advise anyone with a headache that lasts for more than four days after vaccination, or bruising beyond the site of vaccination after a few days, to seek medical attention."