New data links the Pandemrix flu vaccine to an increased risk of sleeping disorder narcolepsy.
Other studies had already shown a link between the drug used in the 2009-10 "swine flu" outbreak and a higher incidence of narcolepsy in vaccinated children, teens and people under 30.
The new research, conducted by Finland's National Institute for Health and Welfare, found that Pandemrix-vaccinated adults between the ages of 20 and 64 had a three-to-five times higher risk of contracting the disease than non-vaccinated people.
Narcolepsy is a chronic disorder of the nervous system that causes excessive drowsiness, often causing people to fall asleep uncontrollably.
Pandemrix was the main vaccine used to fight the 2009-2010 outbreak of H1N1, which infected an estimated fifth of the world's population but turned out to be as dangerous as normal "seasonal" flu.
In Finland, 25 adults developed narcolepsy from 2009 to 2011 -- 23 of them under 40 and none over 64, said the new study.
Eighteen had been given Pandemrix.
"The increased risk of narcolepsy attributed to vaccination with Pandemrix in adults was 1/100,000, whereas in children it was 6/100,000," the institute said in a statement.
The disorder normally occurs among 25-50 of every 100,000 people, although figures are sketchy, according to a recent British study.
In March, Swedish research linked the vaccine to a higher risk of narcolepsy among under-30's, not just children and teens as previously thought.
Sweden's Medical Products Agency found the risk was three times higher among under-20s given Pandemrix, and twice as high in those aged 21 to 30 years.
Neighbours Finland and Sweden recommended their populations of some five and 10 million people respectively, to take part in mass vaccinations during the swine flu scare.
Both governments have since agreed to provide financial compensation for affected children.
Pandemrix, made by British drug company GlaxoSmithKline was the only vaccine used in both countries -- it was given to about 60 percent of the Swedish population.
About 200 children under the age of 19 developed Narcolepsy in Sweden, and 80 in Finland.
But the institute said Pandemrix also prevented an estimated 80,000 swine flu infections in Finland and about 50 deaths.
In February, British research said Pandemrix boosted the narcolepsy risk among four-to-18-year-olds by a factor of 14.
Last September, the European Centre for Disease Prevention and Control (ECDC) said evidence from sleep centres in Finland and Sweden pointed to an added narcolepsy risk of one in 20,000 for Pandemrix-vaccinated children and teenagers.