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.
Compared to normal times, incidence amounted to about 1.6 extra cases per one million people vaccinated, which means the benefit of the jab "greatly outweighed" the risks, they said.
The modest incidence was probably similar to cases of Guillain-Barre that arise from annual vaccination for "seasonal" flu, they said.
The study, published by The Lancet, was carried out by the National Vaccine Program Office in Washington.
It said about 61 million cases of H1N1 were recorded in the United States during the pandemic, leading to 274,000 hospital admissions and 12,470 deaths.
Research published last month estimated that vaccination prevented between 700,000 and 1.5 million cases of flu in the United States, between 4,000 and 10,000 admissions and up to 500 deaths.
The American inoculation programme did not use Pandemrix, a vaccine that uses a booster to prime the immune system against the H1N1 virus.
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.
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.
Health watchdogs anxiously monitor mutations of the flu virus, fearing the emergence of new strains against which humans would have no resistance.
Tens of millions of people were killed in the 1918-19 pandemic of so-called Spanish flu.
But the H1N1 type, while very contagious, was about as lethal as ordinary flu.
Guillain-Barre occurs when the body starts to attack nerves involved in movement and sometimes respiration and other functions.
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.