It’s a mystery that seemingly had no answer: Why did some people develop narcolepsy after being given a vaccine for the dangerous swine flu in 2009?

Now, a team of researchers in the journal Science Translational Medicine has stepped forward with a possible answer that would make Shakespeare proud: the narcoleptics’ own immune system was tricked by the vaccine into attacking the brain cells that control wakefulness.

Beginning in 2009, a pandemic of H1N1 influenza, also known as swine flu, swept across the country. Though not as dangerous as the cousin strain that caused the 1918 pandemic, it’s now believed that the 2009 version ultimately killed between 100,000 to 500,000 people before its end in mid-2010.

At the time, a number of pharmaceutical companies rushed out vaccines intended to prevent or reduce the severity of the pandemic. GlaxoSmithKline released their version, Pandemrix, to various parts of Europe. Before long though, countries like Finland and Sweden began reporting increased rates of narcolepsy, a permanent neurological condition that causes uncontrollable and unpredictable spells of sleepiness, among the Pandemrix-vaccinated, as much as four to nine times higher than normal (the total number of cases still remained extremely small comparatively) Since then, researchers, including those funded by GlaxoSmithKline, have tried to figure out the connection.

This latest study found that a region of protein specific to H1N1 resembled a fragment of the receptor that helps produces hypocretin in the brain, which is usually depleted in those with narcolepsy. They theorized that in these vaccine-associated narcoleptics, Pandemrix triggered a particularly strong immune response to that protein which then bled into a full-on assault on the hapless hypocretin receptors, thus triggering the disorder.

They tested out this theory by testing the blood of 20 such individuals, finding that 17 of the 20 samples contained antibodies that responded to both the flu protein and the receptor. More concretely, they also found that the blood serum of six people who had been given other swine flu vaccines did not possess these antibodies. This might be because the Pandemrix vaccine contains more of the protein than its competitors, the authors said.

Though this current study is obviously very exciting, and its findings may lead to a better understanding of the mysterious by its own rights sleep disorder, it should be noted that scientists have been down this road before and came up short.

In 2013, a team, once again in Science Translational Medicine, published their own small study finding a connection between the flu vaccine and narcolepsy. However, little more than six months later, they were forced to retract the study after they failed to replicate part of their findings — they were unable to find a stronger immune response to hypocretin among narcoleptics compared to a control group.

Though that incident doesn’t mean that the new study is invalid, it does mean that the best approach, as usual, is to wait and see how this research progresses.

Source: Ahmed S, Wayne Volkmuth W, Duca J, et al. Antibodies to influenza nucleoprotein cross-react with human hypocretin receptor 2. Science of Translational Medicine. 2015.