A swine flu jab which was given to hundreds of thousands of children carried an increased risk of developing the sleep disorder narcolepsy, research suggests.
Pandemrix, a vaccine used in response to the swine flu pandemic that began in 2009, increased children's risk of narcolepsy - a chronic disorder which causes excessive daytime sleepiness, research suggests.
For every 55,000 doses delivered around one child developed the condition, health experts said.
The vaccine Pandemrix
At the height of the pandemic, between October 2009 and March 2010, more than 850,000 English children aged six months to 16 years were given the vaccine.
And the jab was also administered to many more people when supplies of the seasonal flu vaccine diminished - 170,000 people, including children, were given the injection between October 2010 to February 2011.
But health experts cautioned that the many children who received the jab are well and are expected to "remain fine" as symptoms appear to develop a few months after the vaccine is given. The injection has not been in use for almost two years.
The research, published in the British Medical Journal, examined 75 children aged between four and 18 who were diagnosed with narcolepsy from January 2008 and who attended sleep centres across England.
Researchers from the Health Protection Agency (HPA) and Papworth and Addenbrooke's hospitals in Cambridge found that 11 of these had received the vaccine before their symptoms began.
After adjusting for clinical conditions, the authors associated the vaccination with a 14-fold increased risk of narcolepsy.
In absolute numbers, this means that one in 52,000 to 57,500 doses are associated with narcolepsy, said the authors.
Since 2011, the use of the vaccine in people under the age of 20 across Europe has been restricted following reports of increased cases of the sleep disorder - which is characterised by periods of extreme drowsiness, sudden naps, and paralysis attacks.
Narcolepsy, which is thought to affect about 20,000 people in the UK, has a genetic component but this has to be triggered by other factors in order for the condition to appear.
Lead author Professor Liz Miller, a consultant epidemiologist with the HPA, said: "These findings suggest there is an increased risk in children of narcolepsy after pandemrix vaccination and this is consistent with findings from studies in other European countries.
"However, this risk may be overestimated by more rapid referral of vaccinated cases. Long term follow up of people exposed to pandemrix is needed before we can fully establish the extent of the association.
"Our findings have implications for the future licensing and use of adjuvanted pandemic vaccines. Further studies to assess the possible risk associated with other vaccines used in the pandemic, including those with and without adjuvants, are also needed to inform the use of such vaccines in the event of a future pandemic."
Study co-author Dr John Shneerson, consultant physician from the Respiratory Support and Sleep Centre at Papworth Hospital in Cambridge, said: "Narcolepsy is thought to be due to a loss of function of a small group of cells in one of the sleep centres in the brain, as a result of an abnormal reaction of the body's immune system.
"Pandemrix may have triggered an immune reaction against the sleep centre cells in those children who were genetically predisposed to develop narcolepsy. This study has been important in helping to shed light on the mechanism of how narcolepsy can develop."
Prof Adam Finn, professor of paediatrics at University of Bristol Medical School, added: "It is now pretty clear that pandemrix, an injected flu vaccine that was used in response to the swine flu pandemic that began in 2009, carried a risk of around one in 50,000 of narcolepsy when given to children, especially teenagers.
"As well as causing suffering and distress to those affected and their families, this raises some important questions about why exactly this happened with this particular vaccine.
"The many children who received pandemrix and are currently well are expected to remain fine as the problem, when it happens, seems to develop a few months after the vaccine and pandemrix has not been in use for two years now.
"Pandemrix is the only vaccine that has been linked to this problem - there is nothing to suggest that it occurs after other flu vaccines or vaccines against other diseases."
A Department of Health spokesman said: "Pandemrix was developed specifically for use in a flu pandemic when the number of lives lost and serious cases could have been enormous.
"The decision to recommend that children got this vaccine during the flu pandemic was based on evidence available at the time, along with the advice from the European Medicines Agency which approved its use.
"We keep all emerging evidence under review and that's why use of pandemrix in those less than 20 years old was stopped in the UK in 2011."