The EJC had been asked to interpret the directive by the French Cour de Cassation, over an action for damages from the family of a man who developed multiple sclerosis shortly after being vaccinated against hepatitis B.
The family brought the action against the manufacturer of the vaccine, Sanofi Pasteur, and three other defendants on the basis of a rule under French law. Under this rule, a causal link can be presumed if a disease occurs shortly after administration of an allegedly defective drug, and if there are no personal or family antecedents related to the disease.
There was no scientific consensus to support a causal relationship between the vaccination and multiple sclerosis, and both national and international health authorities rejected the association between being affected by symptoms characteristic of multiple sclerosis and the vaccination against hepatitis B.
When the case reached the Cour de Cassation, that court asked the ECJ to interpret the directive and to clarify whether it can prevent an applicant from arguing that because a vaccine if defective in one way it is also liable for causing an unrelated problem, without showing scientific proof.
If not, does that mean that liability can be found for any problem what shows indications of causation? And if so, does that mean any causal relationship will always have to be proved scientifically, the court asked.
ECJ advocate general Michal Bobek noted that the standard of proof, and what evidence is sufficient to meet that standard, are not harmonised by the directive.
"In principle, those are therefore questions for national law to resolve," he said.
"I note at the outset that it is not [this] court's role to deduce detailed rules from a directive that sets down only basic rules – but EU law does impose some limits in respect of proof and evidence, which I will develop with a view to assisting the national court in their resolution of the case," he said.
The directive "does not preclude factual presumptions of causal link and effect", he said. These presumptions must respect principles of equivalence and effectiveness, and must be based on relevant and sufficient evidence, and must also be sufficiently rigorous that they do not result in the burden of proof being moved to the defendant.
A court may consider that the facts put forward "constitute serious, specific and consistent presumptions capable of proving the defect in the vaccine, and the existence of a causal relationship between it and the disease, notwithstanding the finding that general medical research does not establish a relationship between the vaccine and the occurrence of the disease", the advocate general said.
This method of proof, however, can only involve presumptions that: rely on evidence that is "both relevant and sufficiently rigorous to sustain the inferences drawn"; that are rebuttable; and that do not unduly prevent the national judge from taking account of relevant evidence.
It must also not prevent the judges from giving due consideration to any medical evidence presented, nor impose a requirement that medical evidence must be presented to demonstrate a defect of a causal link.
A key point considered by the advocate general was the precise meaning of the word 'présomption' in the original French version of the directive, and how this has been interpreted in member states' national law. In this opinion, the term 'presumption' is used "to refer to a situation where a fact or set of facts is established, and from it is inferred the likelihood of another fact or set of facts. In terms of its practical operation, 'presumption' is used here to refer essentially to a form of circumstantial evidence or indirect proof", advocate general Bobek said.
Product safety expert Manoj Vaghela of Pinsent Masons, the law firm behind Out-Law.com said: "This is an interesting opinion, clearly designed to help the unfortunate claimants in this case. English practitioners are likely to focus on identifying whether a defect exists, what that defect is, and the relevant circumstances such as the risk-benefit profile, the regulatory regime, the role of the medical profession and any warnings that were provided."
Opinions of advocates general are not binding on the court, but are followed in the majority of cases.