Europe's drug regulator is reviewing the safety of Glaxosmithkline's Pandemrix swine flu vaccine, which has been given to more than 30 million people in Europe, to examine possible links to a sleep disorder.

The European Medicines Agency (EMA) said it had been asked by the European Commission to conduct the review after a number of cases of narcolepsy were reported in patients who had recently had the jab, mainly in Sweden and Finland. "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 on Friday.

The EMA said its Committee for Medicinal Products for Human Use (CHMP) would examine all the available data to determine whether there is  evidence for a causal link to Pandemrix. 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, but analysts said the review was unlikely to hurt shares in the drugmaker.

"We're talking here about a minute number of people who have had a problem and I don't think there's any risk of the product being pulled," said Mike Ward, a pharmaceuticals analyst at Ambrian Partners in London.

Pandemrix has an adjuvant, or booster, and was not used in the U.S., where the U.S. Food and Drug Administration did not licence any adjuvanted vaccines for use during the H1N1 pandemic.

GSK said in a statement it was conducting its own investigation "in an effort to gather as much additional data as possible regarding the observed cases" and was working closely with regulatory authorities, including the EMA.

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.

The World Health Organisation, which declared H1N1 a pandemic in June 2009 as it spread around the world, said earlier this month that the H1N1 flu pandemic was over and that the global outbreak had turned out to be much less severe than was originally feared.
GSK said that previous experience with large-scale immunisation programs had shown it is likely that a certain number of possible side effects, or adverse events, will be reported among people who receive vaccinations.

"The adverse events that are reported may be the result of underlying conditions, or new conditions that occur in temporal association with the  vaccination, and some events may be related to the vaccine," it said in a statement.