Twelve countries have reported suspected cases of narcolepsy linked to swine flu jabs, the World Health Organisation said on Tuesday as its scientists said the findings warranted more investigation.
The flu vaccine Pandemrix at a chemist shop in Berlin, 2009.
The WHO said in a statement that such sleep disorders, mainly in youngsters, had not been seen with vaccines in the past, and were more frequent in Sweden, Finland and Iceland than in other countries.
Its Global Advisory Committee on Vaccine Safety (GACVS) reviewed data from a Finnish study which found that children injected with the Pandemrix flu vaccine were nine times more likely to contract narcolepsy than those who were not vaccinated.
"The committee agrees that further investigation is warranted concerning narcolepsy and vaccination against influenza (H1N1) 2009 with Pandemrix and other pandemic H1N1 vaccines," the WHO said.
"An increased risk of narcolepsy has not been observed in association with the use of any vaccines whether against influenza or other diseases in the past," it added.
The European Medicines Agency has also launched a probe into the suspected connection.
Narcolepsy is a sleep disorder which causes extreme fatigue and often results in the patient falling soundly asleep without warning, even in the middle of an activity.
"Since August 2010, following widespread use of vaccines against influenza (H1N1) 2009, cases of narcolepsy, especially in children and adolescents, have been reported from at least 12 countries," the WHO said.
A preliminary study by Finland's National Institute for Health and Welfare, THL published a week ago said the most likely explanation of the pattern found in four to 19 year olds was the "joint effect of the vaccine and some other factor(s)."
The Finnish institute stressed that more investigation was needed, but said young people had a "manifold increased risk of falling ill with narcolepsy" if they had been vaccinated with the vaccine made by GlaxoSmithKline.
Last August, THL recommended discontinuing use of that vaccine against A(H1N1) until it could study the signs of a connection.
Pandemrix vaccine was used in 47 countries worldwide during the 2009-2010 season, according to the WHO.