The Oxford-AstraZeneca Covid-19 vaccine trials on children have been put on hold as the UK health regulator investigates a possible link with rare blood clots in adults.

The development comes immediately on the back of the chair of the vaccine evaluation team at the European Medicines Agency (EMA) stating that there is a “clear link” between the Oxford-AstraZeneca vaccine and rare cases of thromboembolic events (blood clots) in the brain, though the causes for this are unknown. The EMA said it would announce findings on Wednesday or Thursday.

The vaccine, which is also being produced in collaboration by the Serum Institute of India, had been previously paused by some European countries following some cases of rare blood clots. The UK regulator, the Medicines and Healthcare Products Regulatory Agency (MHRA), continues to monitor the data and says the benefits of the jab continue to outweigh any risk.

Denmark was the first to temporarily suspend the use of the AstraZeneca vaccine after reports of blood clots in some people. The Nordic nation’s health authority said the decision was based on a precautionary principle and that one person who developed a blood clot after vaccination had died. Norway decided to follow suit and halted use of the Anglo-Swedish company’s vaccine. Italy’s pharmaceutical agency ordered a precautionary ban on a particular batch of the vaccine after what it said were serious adverse events.

The EMA has previously said that there was “no evidence” to support restricting the use of the Oxford-AstraZeneca vaccine in any population. The UK’s MHRA is investigating reports of a very rare and specific type of blood clot in the brain, known as cerebral venous sinus thrombosis (CVST), occurring together with low levels of platelets (thrombocytopenia) following vaccination. It recently said that it identified 30 cases of rare blood clot events out of 18.1 million doses of the jab administered up to and including March 24. There have been seven deaths among those 30 cases.

Our thorough and detailed review is ongoing into reports of very rare and specific types of blood clots with low platelets following the COVID-19 vaccine AstraZeneca. No decision has yet been made on any regulatory action,” said MHRA chief executive Dr June Raine. “People should continue to get their vaccine when invited to do so,” she said 

The regulator maintains that the benefits of the vaccine continue to outweigh any rare concerns that are under investigation for causal links.