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The European Union’s medical regulator said on Wednesday that the AstraZeneca COVID-19 vaccine was “probably the cause” of the very rare, but dangerous, blood-clotting conditions that a handful of people have experienced following inoculation. Moreover, the British medical regulator noted, there’s growing evidence of the link.

In response, the U.K.’s Joint Committee on Vaccines and Immunisation (JCVI) said it is recommending a different vaccine be offered to healthy people between the ages of 18 and 30 if they are not at high risk for COVID-19—that is, for those cases in which another vaccine is readily available.

The European Medicines Agency (EMA) said, given the risks associated with COVID-19, “the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects.” It said national health agencies might choose to restrict use of the vaccine in certain age groups, but that it was not recommending these restrictions at this time.

The findings are another blow to confidence in the AstraZeneca vaccine, which has been plagued by concerns over whether the company and academic researchers behind the inoculation have been fully transparent about both its efficacy and safety, as well as a dispute between the company and European officials over supply shortages.

The unusual, and sometimes fatal, conditions were documented in at least 86 instances, 18 of which were fatal, as of March 22, the European Medicines Agency said in a statement. But this was across 25 million people who’ve been vaccinated with the AstraZeneca jab, meaning that the condition, from the data reported so far, seems to occur in fewer than one person in every 250,000 vaccinated. These are the kind of rare side effects that are unlikely to be picked up even in large-scale clinical trials.

Meanwhile, the EU is mandating that AstraZeneca carry out additional tests to try to determine the exact cause of the rare blood clots and what risk factors may predispose people to developing the condition. The EMA is asking two European universities to carry out studies that look for risk factors for the clotting conditions.

The two rare conditions—cerebral venous sinus thrombosis (CVST), which involves blood clots in veins leading from the brain, and splanchnic vein thrombosis, involving clots in abdominal veins—were both accompanied by low levels of blood platelets and sometimes bleeding, the EMA said.

It said the conditions seemed to occur mostly in women younger than 60, and within two weeks of receiving the first dose of the vaccine. It said the cause of the condition was unclear but might be related to the immune response the vaccine induces.

European medical experts said it was possible the blood clotting could be related to the technology that underpins AstraZeneca’s vaccine. It uses a modified chimpanzee virus, an adenovirus, to carry instructions to the body’s cells to make the spike protein associated with the coronavirus, which in turn prompts an immune response. The Johnson & Johnson vaccine also uses an adenovirus, and European officials said there were hints in the J&J clinical trials, in which a few people also developed blood clots, that clotting might emerge as a potential side effect for that vaccine too. But they said the number of cases in the J&J clinical trials had been too few to make any clear connection at this time.

Drug companies have mostly been exempted from liability in the event that the vaccine they produce does cause harm in rare circumstances as part of their supply contracts with various governments.

Although the conditions are extremely rare, with the EMA advising that people continue to receive the vaccine, the regulator’s finding is likely to further undermine confidence in the AstraZeneca vaccine at a time when Europe has been struggling to roll out inoculations, in part owing to confusion over the effectiveness and safety of the AstraZeneca jab.

Evidence from the U.K., where tens of millions have received the AstraZeneca vaccine, shows that it is safe and seems to prevent severe COVID-19, cutting hospitalizations and deaths. But the vaccine’s reputation has still been marred by a series of communication problems surrounding its clinical trial results, which have raised doubts about how well the vaccine works, particularly in older people, compared with those created by other pharmaceutical companies, such as Pfizer and Moderna.

A large-scale U.S. clinical trial conducted by AstraZeneca was supposed to clear up any lingering concerns about how well the jab works, and, in fact, the company said an analysis of those trial results showed the vaccine was 76% effective at preventing symptomatic COVID-19. But a pall was cast over this seemingly good result when the company received a highly unusual admonishment from the U.S. National Institute of Allergy and Infectious Diseases (NIAID) for possibly releasing “outdated data” in an “interim analysis” of the clinical trial data that pointed to a slightly higher efficacy figure.

AstraZeneca says it is now applying for emergency use authorization in the U.S., but it is unclear if its vaccine will be approved. Even if it is, Anthony Fauci, the head of NIAID, has suggested that the U.S., which has already fully vaccinated about 18% of the population and has approved three other COVID-19 vaccines, may not need the AstraZeneca vaccine.

But the vaccine is still considered essential to ending the pandemic because AstraZeneca has pledged to make billions of doses available at a much lower cost than other vaccine makers. The jab can also be stored at normal refrigerator temperatures, giving it an advantage over some of the other vaccines that must be kept in ultra-freezing conditions.

This story has been updated to clarify that it is the U.K.’s Joint Committee on Vaccines and Immunisation that is recommending adults under 30 years old be offered an alternative vaccine to the AstraZeneca jab, not the British medical regulator, The Medicines and Healthcare Products Regulatory Agency (MHRA).