Following the 2009 swine flu pandemic, roughly one out of 15,000 children in Europe who received the H1N1 vaccine called Pandemrix developed narcolepsy, a disorder that causes irresistible sleepiness. A leading explanation for the rash of narcolepsy cases is that a fragment of the H1N1 virus used to produce Pandemrix may have triggered an autoimmune reaction in some vulnerable children. Researchers have struggled to find evidence for that hypothesis, however. This week, the picture grew murkier when Stanford University researchers retracted a paper in Science Translational Medicine, published 18 December 2013, that was hailed as the first evidence to support the immunological hypothesis. After several attempts, scientists say they are unable to replicate a key part of the experiment.