The risk of a rare immune system disorder following a flu shot appears to be decreasing, a new study shows.

Researchers found the number of cases of Guillain-Barré syndrome (search) that has occurred following vaccination with a flu shot has decreased dramatically during the last 12 years.

Guillain-Barré syndrome is a rare nervous system disorder (search) in which the body's immune system attacks part of the nervous system. This immune system malfunction is usually triggered by an infection or other illness. Guillain-Barré usually occurs a few days or weeks after symptoms of a respiratory or stomach viral infection. Occasionally, surgery or vaccinations will trigger the syndrome.

The link between Guillain-Barré syndrome and the flu vaccination isn't clear. But researchers say concerns about the possible risk of developing Guillain-Barré syndrome after vaccination emerged after an association was noticed during the 1976-1977 "swine influenza" season, and the fact that Guillain-Barré syndrome is the most frequently reported nervous system condition reported after flu vaccination.

Flu Vaccine-Related Guillain-Barré Syndrome Cases Declining

In the study, CDC (search) researchers examined the number of Guillain-Barré syndrome cases reported in adults who received a flu vaccine from July 1990 through June 2003.

During that time, the CDC received 500 reports of Guillain-Barré syndrome following flu vaccination.

They found the annual rates of the disorder decreased from a high of 0.17 cases per 100,000 flu vaccinations in 1993-1994 to a low of 0.04 cases per 100,000 in 2002-2003, which is a fourfold drop.

Guillain-Barré syndrome was confirmed by medical diagnosis in 82 percent of these cases reported to the CDC, and a preceding illness within four weeks of vaccination that may have also triggered the disorder was identified in 24 percent of all cases.

Researchers found there was also a longer delay between the time people received their flu shot and when symptoms of Guillain-Barré syndrome, such as weakness or a tingling sensation in the legs, emerged compared with other reported complications.

For example, symptoms emerged within 14 days of vaccination in 59 percent of all Guillain-Barré syndrome cases compared with 95 percent of all other reported complications.

The results appear in the Nov. 24 issue of The Journal of the American Medical Association.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Haber, P. The Journal of the American Medical Association, Nov. 23, 2004; vol 292: pp 2478-2481. WebMD Medical Reference from Healthwise: "Guillain-BarréSyndrome."