TORONTO—Although there is some evidence of an association between the 2009 H1N1 vaccination and Guillain-Barré syndrome (GBS), this association appears quite rare and the vaccination is still recommended, said Nizar Souayah, MD, at the 62nd Annual Meeting of the American Academy of Neurology.
“This is the first work to suggest that some cases of GBS may be caused by H1N1 vaccination,” said Dr. Souayah, Assistant Professor in the Department of Neurology and Neurosciences at the New Jersey Medical School in Newark. He expressed confidence, however, that “very, very few cases of GBS could be triggered by the H1N1 vaccine” and that “99.9% of patients would benefit” from the vaccine.
James J. Sejvar, MD, a neurologist and epidemiologist at the CDC, emphasized, “The potential risks associated with the H1N1 vaccine are far outweighed by the benefits provided by the vaccine.” The CDC is currently conducting additional research to determine the profile of patients at risk for GBS following H1N1 vaccination.
The possibility that influenza vaccination could lead to GBS was first raised in 1976, when a small but statistically significant association was found between that year’s swine influenza vaccination and the disease. Because influenza did not materialize as a pandemic or epidemic that year, the CDC responded to the association by ceasing its vaccination campaign. Subsequent assessments have failed to demonstrate a consistent pattern of association between GBS and influenza vaccination.
“A Simple Question”
Dr. Souayah first became interested in investigating the possibility of such an association after a patient, whose father had died of GBS-related causes following influenza vaccination, asked him whether she should get a vaccination. “I said I didn’t know,” he explained. “I was not able to answer a simple question.”
As a preliminary step toward an answer, he compared the rate of GBS among patients who received H1N1 vaccination in 2009 to rates of the disease among patients who received seasonal influenza vaccination that year and among patients in the general population. After identifying cases of possible postvaccination GBS by entering GBS-related keywords into the CDC’s Vaccine Adverse Events Reporting System (VAERS) and submitting them to the CDC’s Center for Biologics and Research through a Freedom of Information Act request, he determined whether each case was likely to have been GBS.
A Significant Drop-Off
Dr. Souayah identified 62 cases of GBS following H1N1 vaccination, indicating that the disease occurred in 6.2 per 10 million patients who received the H1N1 vaccine in 2009. In addition, he found 57 cases of GBS following seasonal influenza vaccination, indicating that the disease occurred in 10.6 per 10 million patients who received the seasonal vaccine in 2009. Previous studies have reported that GBS occurs among 34 to 134 people per 10 million people in the general population.
Despite the apparently low rate of GBS among the H1N1-vaccinated population, however, Dr. Souayah said it is “absolutely not” the case that the H1N1 vaccine may protect against GBS. He noted that 60 of the 62 cases of GBS following H1N1 vaccination occurred within six weeks of vaccination, with 51 cases occurring within the first two weeks. The cases peaked in the second week after vaccination and decreased during each of the next six weeks.
“If you look at the [temporal] distribution, you see this curve. That curve is not normal,” said Dr. Souayah. The odds of such a distribution occurring by chance are “almost impossible,” he said, at less than 0.00002. He reported a similar temporal distribution among the 57 GBS cases following seasonal vaccination, with 51 of these cases occurring within the first two weeks after vaccination.
Dr. Souayah added that “the actual incidence of GBS after H1N1 vaccine is probably higher than my reported rate,” because VAERS is a passive reporting system. The CDC “depends on the reports from physicians, from industry, and from the lay public to bring what they suspect might be adverse events to [its] attention,” for inclusion in VAERS, Dr. Sejvar explained.
Asked if certain characteristics put patients at risk for GBS after H1N1 vaccination, Dr. Souayah echoed his earlier response to a patient: “The answer is that I don’t know.” He added that prospective, case-controlled studies are needed both to prove the apparent association between the vaccination and the disease and to determine a profile for at-risk patients.
“I would love to do a prospective study,” Dr. Souayah said. “For each patient vaccinated, you give [him or her] an application to fill out”a profile. I have a plan for that. It’s not really impossible. We just need to be willing to do that.”