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 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.
However, the UN health agency decided to keep its advice in favour of vaccination, including with the Pandemrix vaccine highlighted in the study, because it still felt the benefits outweighed a relatively small risk, spokeswoman Alison Brunier said.
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.
The Pandemrix vaccine made by GlaxoSmithKline was used in 47 countries worldwide during the 2009-2010, according to WHO, and was included by the agency in donations made to poor nations during the flu pandemic.
The WHO Global Advisory Committee on Vaccine Safety (GACVS) last week 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 (EMA) has also launched a probe into the suspected connection.
"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 added.
Brunier noted that the EMA -- the regulatory body responsible for approving the vaccine -- had not taken any action. "There's no change to the WHO's current position on use of pandemic influenza vaccines," she added.
"This means that countries should continue vaccinating against H1N1 to immunise persons at risk of severe disease, using monovalent vaccines including Pandemrix if there is no trivalent seasonal vaccine available," she added.
Monovalent vaccines target a single strain of flu while trivalent vaccines, more commonly used every flu season, are effective against three strains.
The 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)."
It stressed that more investigation was needed.
Last August, THL recommended discontinuing its use against A(H1N1) until it could study the signs of a connection.