A swine flu vaccine given to millions of Britons is under review over a possible link to the sleeping disorder narcolepsy, it was announced today.
The European Medicines Agency is looking into Pandemrix, made by GlaxoSmithKline, after a number of cases were reported, mostly in Sweden and Finland.
Pandemrix was the main vaccine used in the UK at the height of last year's flu pandemic, and was given to millions of people in high-risk groups, including children and those with conditions such as diabetes, heart disease and asthma.
Overall, it has been given to at least 30.8 million Europeans.
Narcolepsy is a rare sleep disorder which causes a person to fall asleep suddenly and unexpectedly.
In a statement, the European Agency said: "Although the cases of narcolepsy have been reported in temporal association with the use of Pandemrix, it is at present not known if the vaccine caused the disorder.
"The Agency's Committee for Medicinal Products for Human Use (CHMP) will look carefully at all of the available data to determine whether there is evidence for a causal association."
On Tuesday, Finland's National Institute for Health and Welfare recommended that vaccination with Pandemrix be stopped until the suspected link with narcolepsy had been thoroughly evaluated.
In December, the European Medicines Agency expressed concerns that children having their second dose of Pandemrix may develop a high fever.
Parents and doctors were being urged to keep watch on a child's temperature after research found they were more likely to suffer a fever - above 38C (100F) - following the second dose compared with after the first.
Youngsters were also more likely to suffer soreness at the site of injection as well as drowsiness, irritability and loss of appetite after the second dose.
A spokesman for the Medicines and Healthcare products Regulatory Agency (MHRA) said it was "aware of the case reports of narcolepsy" and was "evaluating these in collaboration with other EU authorities".
"Narcolepsy is a rare, natural illness, with around 10 new cases per million people every year and at present no link with the vaccine has been established," he added.
"After use of more than six million doses of swine flu vaccine in the UK, no cases of narcolepsy have been reported following vaccination in the UK.
"Pandemrix vaccine remains available for use as recommended."