Vaccination in the United States against H1N1 "swine" flu, which swept the world in 2009-10, carried a small but tolerable risk of Guillain-Barre syndrome, a disorder that can cause muscle weakness or temporary paralysis, a study published on Wednesday said.
US researchers scanned a database of side effects from the campaign in which 23 million Americans were inoculated with a so-called monovalent inactivated vaccine against H1N1.
They recorded 77 cases of Guillain-Barre that occurred within three months of a vaccination.
According to investigations in Finland, Sweden and England, Pandemrix increased the risk of narcolepsy, a disorder that causes extreme drowsiness, among teenagers and young people, although the extent of the problem is unclear.
It leapt into the headlines in 1976, when a campaign to vaccinate the US population against a new flu strain was stalled by contentions - later contested - that the jab led to an increase in cases of this otherwise rare auto-immune disorder.