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How the Swine Flu Vaccine Provides Insight Into Narcolepsy

In 2009, doctors noticed that children who got a particular strain of swine flu vaccine were suddenly much more likely to develop narcolepsy

Rose Eveleth

Contributor

December 20, 2013

vaccine — Courtesy of Flickr user Harald Groven

You might not see an immediate connection between swine flu and narcolepsy. Doctors didn't either. But in 2009, doctors suddenly noticed that children who got a particular strain of swine flu vaccine were suddenly much more likely to develop narcolepsy. This, alongside the fact that China saw narcolepsy increase four times over during their swine flu pandemic, has stumped doctors. How could the two be linked? One recent study might have the beginnings of an answer.

Emmanuel Mignot at Stanford University recently looked at a certain type of immune cells called C4 cells in children who received a swine flu vaccine called Pandemrix in 2009. He then compared those results with the C4 cells of narcoleptic children. What he found was that in children with narcolepsy, the CD4 cells responded to two things—hypocretin and a surface protein in the flu virus. Hypocretin is a neurotransmitter that signals to the brain that it is awake, and it's diminished in those with narcolepsy. In children without narcolepsy, the CD4 cells didn't react to either.

Debora MacKenzie at New Scientist explains his results:

Narcoleptic children who were given ordinary 2012 flu vaccine – which, like Pandemrix, contains the HA protein from the 2009 virus – also responded with a surge in CD4s that attack hypocretin and the cells that make it. So immunity to the 2009 HA protein, either in Pandemrix or in flu itself, says Mignot, had unexpected consequences for hypocretin production.

Essentially, these children's immune systems were mistaking hypocretin for a viral protein, MacKenzie explains. But why that happens in some children and not others is still unclear. For scientists, this direct link between an environmental factor and an autoimmune disease is particularly interesting. In most cases, environmental factors are hard to parse and even harder to directly blame. But for narcolepsy and swine flu, things seem to be relatively clear.