A new study offers clues as to why a swine flu vaccine given to 6 million people in Britain was found to have triggered narcolepsy in rare cases. Scientists believe that the Pandemrix vaccine, manufactured by GlaxoSmithKline PLC (GSK), could harm a critical portion of the brain used for regulating sleep.

Pandemrix was recommended by the U.K. government during the 2009-2010 swine flu outbreak, but was withdrawn after medical records showed a growing number of narcolepsy cases among those who received the vaccine. It was unclear why the vaccine caused the chronic illness in about one in 55,000 recipients. The vaccine was never used in the United States.

Researchers reported on Wednesday that the H1N1 flu strain contains a protein that is similar to a brain receptor that regulates wakefulness by producing the hormone hypocretin, and that the antibodies triggered by the vaccine could have destroyed these neurons as well. The results of their research were published Wednesday in the journal Science Translational Medicine.

“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,” Sohail Ahmed, global head of clinical sciences at Novartis Vaccines, and the lead author of the study, said, according to the Guardian.

A GSK spokeswoman told Reuters that the company would review the study. "We are actively conducting research into the observed association between Pandemrix and narcolepsy and the interaction this vaccine might have had with other risk factors in those affected," she said.

Narcolepsy is an incurable, chronic brain disorder marked by excessive daytime sleepiness and possible severe nightmares. Other countries where Pandemrix was used, including Britain, Finland, Sweden and Ireland, have also reported a rise in narcolepsy cases.

In June, a 12-year-old boy was awarded $187,000 after he successfully sued the British government for damages incurred by Pandemrix-induced narcolepsy. The ruling is expected to open doors for as many as 100 other families to receive compensation.