The Curious Link Between H1N1 Flu and Narcolepsy

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A Chinese nurse inoculates a man with the swine flu vaccine at a hospital in Hefei, east China's Anhui province on November 10, 2009.

A swell in new cases of narcolepsy in China followed seasonal patterns of flu, including H1N1, according to a recent study led by Dr. Emmanuel Mignot of the Stanford University School of Medicine. The new cases appear to be associated with flu infection itself, not with flu vaccinations.

A peak in cases of narcolepsy — an autoimmune disease that causes people to fall asleep suddenly — occurred about five to seven months after a peak in cold and flu cases in the country, the study found. Onset in the spring was seven times more common than in the winter.

Researchers believe that people have a genetic predisposition to narcolepsy, which may be triggered by some environmental factor, such as an upper airway infection. Reported the New York Times:

Narcolepsy and a related but even rarer illness, cataplexy — a tendency to collapse when swept by strong emotions — are caused by the death of brain cells that secrete hypocretin, which regulates sleep. Those cells, Dr. Mignot explained, are probably killed by autoimmune reactions that stem from winter infections like flu and strep throat.

For several years after the 1918 Spanish flu pandemic, medical authorities described a seasonal somnolence they called “encephalitis lethargica.”

The new study looked at data on 900 cases of narcolepsy diagnosed in Beijing between September 1988 and February 2011. The researchers conducted phone interviews with 154 patients whose narcolepsy appeared after the 2009-10 H1N1 pandemic, asking about their history of flu, H1N1 vaccination and other diseases.

Following the pandemic, rates of narcolepsy tripled, compared with other years. But fewer than 6% of the interviewed patients had received a flu vaccination, suggesting that their narcolepsy was unrelated to vaccines.

Last year, European research found that an H1N1 vaccine called Pandemrix was associated with a ninefold increase in narcolepsy cases in children in Finland. But that vaccine contains adjuvants that boost the drug’s potency and the body’s immune response to it; the additives are not included in vaccines used in the U.S. and China.

The findings suggest that getting vaccinated against H1N1 and avoiding infection with the flu may help protect patients who have a genetic susceptibility to narcolepsy. It’s very possible that being vaccinated with a mild vaccine, one without the adjuvants in question, blocks you from getting a big infection that could increase your risk of narcolepsy,” Mignot said in a statement.

The study appears in the Annals of Neurology.

Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.

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