Sweden reported 60 cases of narcolepsy after people were vaccinated with Pandemrix, manufactured by pharmaceutical giant GlaxoSmithKline. Many of the cases involved children.
Narcolepsy is a rare sleep disorder that causes a person to fall asleep suddenly and unexpectedly. According to the WHO, the rates reported in Sweden, Finland and Iceland were notably higher than those in other countries.
Studies are ongoing to determine if the apparent increased risk of narcolepsy reported in Sweden is higher in vaccinated persons, the WHO said in a statement on Tuesday.
According to Sweden's National Board of Health and Welfare (Socialstyrelsen), 31 million people have been vaccinated with Pandemrix in Europe. A Finnish study released last week found that the risk of children suffering from narcolepsy was nine times higher for those vaccinated with Pandemrix.
However, the WHO has not noticed an increased risk of narcolepsy in association with the use of any vaccines, both against influenza and other diseases in the past.
Sweden's health board has maintained close contact with the Medical Products Agency (Läkemedelsverket, MPA).
"We will closely follow the MPA's investigation and listen to their assessment. The Finnish study must be seen as part of a large investigation currently being conducted in Europe," Anders Tegnell, department head at the health board, said in a statement last week.
The board continues to recommend that groups at increased risk of serious illness receive regular influenza vaccinations, which protect against three types of influenza, including H1N1.
"Given the current situation, there is no reason to change our vaccination recommendations. Pandemrix is recommended based on an individual assessment by the attending doctor for reduced immune defence individuals who need greater protection, such as those who have recently received a transplant," said Tegnell.