A flu vaccine in Europe used during the 2009 H1N1 swine flu outbreak has been linked to narcolepsy. Now, a new study published in Science Translational Medicine last July 1 has provided some clues as to how this side effect came to be.

People who have narcolepsy tend to be more sleepy during the day, and they suddenly fall asleep for a rather brief period of time. It has been established that narcoleptics lack cells responsible for producing hypocretin, a neurotransmitter that regulates sleep-wake cycles.

If a person lacks that neurotransmitter, he or she has narcolepsy, said Stanford University professor of psychiatry and behavioral sciences Emmanuel Mignot.

Pandemrix, the flu vaccine involved, has been found to be linked with narcolepsy cases in previous studies. For instance, a previous study found that narcoleptic British children were 14 times more likely to have received Pandemrix compared with other children, according to a report from LiveScience.

To find the connection, researchers analyzed constituents of the vaccine and compared it with that of another swine flu vaccine from Novartis called Focetria.

Researchers discovered that a viral protein present in Pandemrix mimicked the structure of the hypocretin receptor. Furthermore, it was found that the Pandemrix vaccine had more levels of the viral protein compared with Focetria.

Researchers also theorized that the reaction may have something to do with genetics. Specifically, researchers speculate that people at risk of developing the sleep disorder due to genetic mutation who received Pandemrix had their autoimmune response triggered by the vaccine.

For this part of the study, researchers examined blood from 20 people who received Pandemrix and developed narcolepsy. It was found that their blood samples had antibodies that bind to both H1N1 virus and hypocretin receptor--findings not seen in Focetria.

The high count of antibodies not only attacks the flu virus but also the hypocretin receptor, consequently disrupting hypocretin production, said Lawrence Steinman of Stanford University, one of the researchers.

Researchers say that more work is needed to confirm the latest findings, but according to infectious disease specialist Amesh Adalja, the explanation provided seems to be a plausible one.

"It seems to me, that this really is a likely explanation, based upon all these steps the researchers took to prove this hypothesis," said Adalja, who was not involved in the study.