A vaccine used in Sweden in the 2009-2010 "swine flu" pandemic is linked to a higher risk of the sleeping disorder narcolepsy among under 30s, and not just children and teens as previously thought, a Swedish study showed Tuesday.
Risk of narcolepsy was three times higher among those under the age of 20 who were given the Pandemrix vaccine compared with those who were not inoculated, and twice as high in those aged 21 to 30 years, the Medical Products Agency said.
The risk declined gradually with age and was non-existent by the age of 40.
In real terms, the risk of under 20s developing narcolepsy was an extra four cases per 100,000 vaccinated people per year, and for 21-30 year olds, an extra two cases per 100,000, the author of the study, Ingemar Persson, said.
"We're talking about a horribly debilitating disorder, and that's too many cases caused by a vaccine of all things," he said.
Narcolepsy is a chronic disorder of the nervous system that causes excessive drowsiness, often causing people to fall asleep uncontrollably.
It normally occurs among 25-50 of every 100,000 people, although figures are sketchy, according to a recent British study.
Pandemrix, made by British drug company GlaxoSmithKline, was given to around 60 percent of the Swedish population during the 2009-2010 swine flu pandemic.
In Sweden, some 200 children under the age of 19 developed narcolepsy after receiving Pandemrix, while in neighbouring Finland, which also used the same vaccine, some 79 children were diagnosed with the disorder.
Both Finland and Sweden recommended their populations, of around five and 10 million respectively, to take part in mass vaccinations during the swine flu scare. Pandemrix was the only vaccine used in both countries.
Both governments have agreed to provide financial compensation for the affected children after their own national research showed a link between the inoculation and narcolepsy.
Sweden's Pharmaceutical Insurance agency said it would now examine whether to provide compensation to some 90 young adults who have developed narcolepsy.
"The narcolepsy problem came as a total surprise and since it's such a rare disease it wasn't something we could have looked into beforehand," professor Persson said, defending Sweden's mass vaccination programme.
Pandemrix uses an adjuvant, or booster, called AS03, which aims to strengthen the immune response to the H1N1 virus.
More than a fifth of the world's population was infected with the H1N1 virus in the 2009-2010 pandemic, according to estimates published in January.
Explore further: Swine flu vaccine linked to child narcolepsy: EU watchdog