The highest court of the European Union ruled Wednesday that courts can consider whether a vaccination led to someone developing an illness even when there is no scientific proof.

The decision was issued on Wednesday in relation to the case of a Frenchman known as Mr. J.W., who was immunized against hepatitis B in late 1998-99. About a year later, Mr. J.W. was diagnosed with multiple sclerosis. In 2006, he and his family sued vaccine-maker Sanofi Pasteur in an attempt to be compensated for the damage they claim he suffered due to the vaccine. Mr. J.W. died in 2011.

France's Court of Appeal ruled there was no causal link between the hepatitis B vaccine and multiple sclerosis, and dismissed the case. Numerous studies have found no relationship between the hepatitis B shot and multiple sclerosis.

After the case went to France's Court of Cassation, it was brought to the European Union. On Wednesday, the EU's top court said that despite the lack of scientific consensus on the issue, a vaccine could be considered defective if there is "specific and consistent evidence," including the time between a vaccine's administration and the occurrence of a disease, the individual's previous state of health, the lack of any family history of the disease and a significant number of reported cases of the disease occurring following vaccination.

In a statement, the court said that such factors could lead a national court to conclude that "the administering of the vaccine is the most plausible explanation" for the disease and that "the vaccine therefore does not offer the safety that one is entitled to expect." It did not rule on the specific French case.