The World Health Organization says genetic factors may have contributed to a spike in narcolepsy cases observed among children in Finland who received the swine flu vaccine Pandemrix.
A WHO expert panel says of 22 narcolepsy patients tested, all had a gene associated with the disorder.
Narcolepsy causes people to suddenly fall asleep but is rarely fatal.
WHO's Global Advisory Committee on Vaccine Safety says at least 12 countries told the Pandemrix's British manufacturer GlaxoSmithKline of narcolepsy cases following vaccination.
The panel said Tuesday that incidence rates in Sweden, Finland and Iceland were "notably higher" than in nine other countries it declined to identify.
WHO continues to recommend the use of Pandemrix for children.