Around a million children in Britain received the Pandemrix vaccine during the swine flu pandemic from 2009 to 2010.
Now scientists in Cambridge say they've discovered evidence the jab may have triggered the sleep disorder narcolepsy in up to 20 of those children.
It's believed the vaccine brought on narcolepsy in children who were predisposed to it rather than caused the disease - but that's small comfort to families.
The Sleep Centre at Papworth Hospital is the largest in the UK. Around 300 patients with narcolepsy are treated here. The centre analysed all children in Britain who developed narcolepsy during the swine flu pandemic.
Mindful of the panic once caused after the MMR vaccine was wrongly linked to autism, scientists stress that their research did not show any safety concerns with seasonal flu jabs and only related to Pandemrix.
The vaccine's manufacturer, GSK, said it was conducting their own research into the link between narcolepsy and their vaccine.
"We currently believe that the available data are insufficient to assess the likelihood of a causal association between Pandemrix and narcolepsy."