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, said health experts.
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.
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."
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.