Vaccines used in the 2009 "swine flu" pandemic caused no perceptible rise in a neurological disorder called Guillain-Barre syndrome, a study published in the British Medical Journal (BMJ) said on Wednesday.
The investigation is part of a safety probe into booster-aided flu vaccines that dates back 35 years.
In 1976, immunisation against a "swine flu" virus in the United States was halted after doctors found a seven-fold increase in risk from Guillain-Barre.
Studies on subsequent seasonal virus vaccinations found either no increases or only a modest increase in risk, leaving unresolved a debate whether the flu jab could be dangerous or not.
Guillain-Barre is a disease in which the body's immune defences attacks nerve cells, causing muscle weakness and sometime paralysis. It is very rare, occurring in a range between 0.4 and four per 100,000 "person years," a measure of risk used in epidemiology. It is fatal in three to 10 percent of cases.
The study, funded by the European Centre for Disease Prevention and Control (ECDC) was done among 50 million people in five European countries where doctors had used a flu jab with a booster, also called an adjuvant, to prime the immune system.
The probe matched 104 cases of Guillain-Barre syndrome against people who were otherwise healthy.
They found no evidence of risk of Guillain-Barre syndrome from the vaccination.
However, they could not rule out a theoretical but still small increased risk, amounting to less than three excess cases of the syndrome for every million individuals protected by the vaccination.
"This study provides reassurance that adjuvanted pandemic influenza A(H1N1) 2009 vaccines did not increase the risk of Guillain-Barre syndrome substantially, if at all," says the paper.
Swine flu killed at least 18,449 people and affected some 214 countries and territories after it was uncovered in Mexico and the United States in April 2009.
The UN's World Health Organisation (WHO) declared a pandemic on June 11, 2009. The event was formally over on August 10, 2010.