The number of people in the U.S. who developed the mysterious neurological disease Guillain-Barré Syndrome after receiving a vaccination against H1N1 was low, a new report shows.

According to research presented at the American Academy of Neurology's 62nd Annual Meeting in Toronto, there were 35 reports of GBS following H1N1 vaccination in the U.S. by the end of 2009. That amounts to 3.5 reports of GBS per 10 million people vaccinated.

The rate of GBS in the general population is estimated to be between one to four cases per 100,000 persons per year.

GBS is a rare disorder in which the body's immune system attacks part of the nervous system, causing weakness and sometimes temporary paralysis of the arms and legs. It's not fully known what causes the syndrome, but about two-thirds of people who develop the condition do so several days or weeks after they have been sick with diarrhea or a respiratory illness.

A massive vaccination program against another form of flu in 1976 led to a disturbing number of reports of GBS cases and caused health authorities to halt the vaccine program.

No other flu vaccines have ever been clearly linked to GBS. It was not anticipated that the 2009 H1N1 vaccine would increase the risk either.

Scientists analyzed information obtained from the U.S. Centers for Disease Control and Prevention and U.S. Food and Drug Administration Vaccine Adverse Event Reporting System (VAERS) say that among 33 patients who were hospitalized with GBS, there was one report of death and one of permanent disability.

The number of GBS cases reported by the same researcher was only slightly higher after seasonal flu vaccination in 2009: 57 reports of GBS were received by VAERS , an estimated rate of 7.3 reports of GBS per 10 million vaccinations.

"Although preliminary, these reported cases of GBS do not appear to show an increased risk of GBS following vaccination with either the 2009 H1N1 or the seasonal flu strain and the safety record for these vaccines is excellent," said study author Dr. Nizar Souayah, with New Jersey Medical School in Newark.

"CDC, FDA and neurologists around the world are continuing to closely monitor people after vaccination for this disease."

Since VAERS receives voluntary reports of adverse events from manufacturers, providers, vaccines, and caregivers, cases of illness may be either over or underreported, and calculation of actual rates is not possible.

VAERS cannot determine cause-and-effect, and an adverse event report only indicates that the event occurred sometime after vaccination.