Pandermrix was the vaccine used to fight off swine flu during the height of the H1N1 pandemic in 2009 and 2010. It was never administered in the United States, but European countries like Finland and the U.K. did use it for high risk patients, according to The Guardian. However, cases of narcolepsy spiked following this and the drug has been linked to uncontrollable daytime sleepiness among 55,000 recipients.

A team of international researchers has now identified what may have happened. A protein in the vaccine seemed to have mimicked an element in the brain that plays a vital role in the sleep disorder. This protein is believed to have latched on to the brain receptors, thus damaging it. The drug may have also produced antibodies that affected and interfered with how the brain cells regulated tiredness.

"There's a lot of evidence now to suggest that narcolepsy is an autoimmune disease. In our study, we found antibodies that could cross-react to both the flu protein produced by the vaccine and receptors on the neurons," said Dr. Sohail Ahmed, one of the authors of the study, via The Guardian.

Blood samples from 20 people have been studied in the research and were found to have harbored the antibodies. The researchers compared this with other blood samples treated with another type of vaccine, Focetria, and the antibodies didn't register. Pandermrix apparently contained more flu protein than Focetria.

Dr. William Schaffner, an expert in flu vaccines but who wasn't part of the researching team, said that the results were plausible. "This would also appear to be a solvable problem," he told Fox News. He suggested that future vaccines can still work against swine flu and are in fact really important, but these must not contain high doses of "cross-reactive protein" to reduce the side effects.

The study was published in the journal Science Translational Medicine.