Europe's drug regulator is reviewing the safety of GlaxoSmithKline's Pandemrix flu vaccine, which has been given to more than 30 million people in Europe, to investigate possible links to a sleep disorder.
The European Medicines Agency said it had been asked by the European Commission to conduct the review after a number of narcolepsy cases were reported, mainly in Sweden and Finland.
Pandemrix has been used since September 2009 for vaccination against H1N1 pandemic flu, also known as swine flu, and has been given to at least 30.8 million Europeans.
Narcolepsy is a rare sleep disorder that causes a person to fall asleep suddenly and unexpectedly. Its precise cause is unknown, but it is generally considered to be triggered by a combination of genetic and environmental factors, including infections.
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 EMA drugs regulator said in a statement.
It said its Committee for Medicinal Products for Human Use (CHMP) would examine all of the available data to determine whether there is evidence for a causal link.
Glaxo shares were steady and 0.3 percent higher on the day at 1225 GMT.