The European Medicines Agency said Thursday it had found no link between the Pandemrix swine flu vaccine and the sleeping disorder narcolepsy amid patient concerns, notably in Scandinavia.
The EMA "concluded that the available evidence was insufficient to determine whether there is any link between Pandemrix and reports of narcolepsy, and that further studies were necessary to fully understand this issue", a statement said.
"The ongoing review is complex and will take some three to six months to complete," the London-based body added.
Narcolepsy is a condition in which sufferers suddenly fall into a deep sleep.
Pandemrix - produced by British pharmaceutical company GlaxoSmithKline - has been used since September 2009 for vaccination against H1N1 influenza.
The EMA added on Thursday that it had so far received 81 reports from healthcare professionals suggestive of narcolepsy, including 34 from Sweden, 30 from Finland and 10 from France.