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 Pandemrix was recommended by the U.K. government during the 2009-2010 swine flu outbreak, but was withdrawn after medical records showed a growing number of narcolepsy cases among those who received the vaccine.Researchers believe that the vaccine could harm a critical portion of the brain used for regulating sleep. The vaccine caused chronic illness in about one in 55,000 recipients. The study is printed in the journal Science Transitional Medicine.