Scientists investigating why a GlaxoSmithKline flu vaccine triggered narcolepsy in some people say they have the first solid evidence the rare sleep disorder may be a so-called “hit-and-run” autoimmune disease.

The researchers were trying to find out why, of two different flu vaccines widely deployed during the 2009/2010 swine flu pandemic, only one — GSK’s Pandemrix — was linked with a spike in cases of narcolepsy.

In a study published in the journal Science Translational Medicine, they said the answer could lie in a protein in the H1N1 flu strain found in high amounts in the GSK shot but at much lower levels in the other vaccine, Novartis’ Focetria.