Scientists offer explanation for narcolepsy side effect of flu vaccine

One vaccine used in Europe during the 2009 swine flu pandemic was linked to rare cases of a baffling side effect - the sleep disorder narcolepsy. Now new research offers a clue to what happened.

The vaccine Pandemrix was pulled off the market amid reports of narcolepsy in Finland and elsewhere that sparked questions globally about flu shot safety.

On Wednesday, an international team of researchers reported the problem might have been a case of mistaken identity by the immune system.

Narcolepsy is an incurable disorder that interferes with normal sleep cycles, leaving people chronically sleepy during the daytime and apt to abruptly fall asleep. No one knows what causes it, although patients have very low levels of a brain chemical named hypocretin that's important for wakefulness.

In the new study, Dr Lawrence Steinman of Stanford University and colleagues found that the H1N1 virus contains a protein that is structurally similar to part of a brain cell receptor for that wakefulness chemical. They wondered if the flu-fighting antibodies generated by the Pandemrix vaccine might also latch onto those narcolepsy-linked receptors, leading to damage.

Colleagues in Finland sent blood samples that had been stored from 20 people diagnosed with vaccine-associated narcolepsy. Sure enough, 17 harboured antibodies capable of reacting both to flu and to those narcolepsy-linked receptors, Steinman's team reported in the journal Science Translational Medicine.