Boris Johnson has sought to reassure people about the safety of the Oxford/AstraZeneca Covid vaccine as a trial in children was paused while regulators investigate rare reports of blood clots, largely in younger women. The prime minister urged the public to take the jab when it is offered, while scientists stressed the side-effects were extremely rare and the benefits of protection against coronavirus were great. Some UK drug safety experts believe there could be a causal link between the AstraZeneca jab and rare blood clotting events including cerebral venous sinus thrombosis (CVST). But they said vaccination programmes must continue, with risk mitigation for women under 55. Doctors have already been alerted to CVST symptoms, which include headache, blurred vision and fainting. Oxford University is running a trial in more than 200 children and young people aged six to 17 to see whether they could benefit from the AstraZeneca jabs. The trial was paused on Tuesday as a precautionary measure in response to investigations by the Medicines and Healthcare products Regulatory Authority (MHRA) in the UK and the European Medicines Agency (EMA), a university spokesperson said. The regulators are considering whether any action should be taken, with statements expected within days. The Oxford spokesperson added: “While there are no safety concerns in the paediatric clinical trial, we await additional information from the MHRA on its review of rare cases of thrombosis/thrombocytopenia that have been reported in adults, before giving any further vaccinations in the trial.” On a visit to the AstraZeneca manufacturing plant in Macclesfield, Cheshire, on Tuesday, Johnson said that getting the vaccine was “the key thing”. The jab has been given to more than 18 million UK adults with just 30 rare blood clotting cases reported, and seven deaths. “The best thing people should do is look at what the MHRA say, our independent regulator – that’s why we have them, that’s why they are independent,” said Johnson, who has received a first dose of the vaccine himself. “Their advice to people is to keep going out there, get your jab, get your second jab.” Prof Saad Shakir, the director of the drug safety research unit (DSRU) at Southampton University, said on Tuesday that the evidence accumulated in Europe and the UK of links between the vaccine and the rare blood clots “is consistent with causality”. While the dangers of coronavirus were so great that vaccination must not stop, he said, measures should be put in place to reduce any extra risk to women under the age of 55, who seemed to be most affected. The DRSU has shared its analysis with the regulators. Earlier on Tuesday, the EMA denied it had already established a causal connection between the vaccine and the clots, after a senior official from the agency said there was a link. Marco Cavaleri, the EMA’s head of vaccines, had earlier told Italy’s Il Messaggero newspaper that in his opinion “we can say it now, it is clear there is a link with the vaccine … but we still do not know what causes this reaction”. Across Europe, some countries have already decided to give the AstraZeneca jab only to older people – over-60 in Germany and over-55 in France – while in others, the use of the vaccine is still suspended. The DSRU at Southampton University looked at cases of thrombosis (blood clotting inside the arteries) linked to thrombocytopenia (a reduction in blood platelets that usually causes bleeding but in rare cases results in clotting) and concluded that they were linked to the AstraZeneca vaccine. The events are very rare, however. In the UK, as of 24 March, 30 events had occurred resulting in seven deaths from 18.1m doses of vaccine, they said. In Germany, there was one event of cerebral venous thrombosis for every 46,512 women vaccinated and one female death associated with this condition for every 149,860 vaccine doses given to women of any age. Even for younger people, the risk of death from Covid is higher. In the UK, according to the scientists, it has been calculated that 47,000 vaccines prevent one death from Covid among all people under 50. Shakir says that all the cases now in the public domain occurred within four to 16 days of vaccination. “So, there is what we call a close temporal relationship, and they don’t seem to be events of Covid, which you get in the first two weeks after vaccination,” he said. “The second thing is that there is a clear clinical description and similarities between the cases. The thromboses, lowering of the blood platelets, and various haematological changes. All of them are consistent with an event, which occurs very, very rarely, and certainly only with a drug called heparin.” Heparin is a blood-thinning drug. Very occasionally, it causes a syndrome called HIT – heparin-induced thrombocytopenia. A group of German scientists led by the clotting specialist Andreas Greinacher of the University of Greifswald has already pointed out that the blood clotting events reported after the AstraZeneca jab look very similar to HIT. Shakir said the AstraZeneca vaccine was safe and effective. “It has protected millions of people from Covid-19 and will continue to do so around the world,” he said. Many vaccines in widespread use have side-effects, he said. A flu vaccine can in rare cases cause Guillain-Barré syndrome, for instance, in which the body’s own immune system attacks the nerves and can cause paralysis. The answer is not to stop using the vaccine, but to mitigate the risk by assessing which people are most likely to get the side-effect, looking at any previous illnesses, medication use and their family history, for instance. Regulators are now looking at this and also at any symptoms which might enable people experiencing the rare blood clots to be identified early and treated before their condition becomes too severe. The MHRA said it was still considering the evidence. “People should continue to get their vaccine when invited to do so,” said Dr June Raine, its chief executive. “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.”