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Influenza Vaccination Linked to Very Small Risk of
One of 2 population-based studies conducted by researchers at the Institute of Clinical Evaluative Sciences, in Toronto, Ontario, found individuals who received the vaccine were almost 1.5 times more likely to develop GBS 2 to 7 weeks immediately following vaccination compared with the control interval of 20 to 43 weeks.
However, a second study, which used time-series analysis to determine whether the institution of a universal influenza immunization program was associated with subsequent GBS hospital admissions, showed no statistically significant increase.
“Although the relative risk of developing Guillain-Barré syndrome immediately following influenza vaccination is about 1.5, the absolute risk is very small and amounts to 1 or 2 people per million,” report coauthor Kumanan Wilson, MD, told Medscape.
The study is published in the November 13 issue of Archives of Internal Medicine.
The investigators used data from a universal influenza immunization program in the province of Ontario. This annual initiative, which is one of the largest universal vaccination programs in the world, was adopted in 2000 and offers all residents over the age of 6 months free influenza vaccines.
In the first study, which was a self-matched case-series design, investigators identified 1601 patients aged 18 years and older with incident hospital admission for GBS from April 1, 1993 to March 31, 2004. They then identified which of these patients had received influenza vaccines and compared their risk for GBS hospitalization 2 to 7 weeks after vaccination with the time period 20 to 43 weeks later.
Of these cases, 269 occurred within 43 weeks of vaccination. This follow-up period was then divided into 8 distinct time intervals, and it was shown that subjects were 1.5 times more likely to develop GBS in the 2 to 7 weeks following vaccination than they were at any other time during the follow-up period.
Very Small Absolute Risk
“The results [of the first study] did show an increased risk relative risk of GBS. But in order to put this finding into context, we conducted a second study to see whether it had an actual impact at a population level,” said Dr. Wilson.
To obtain this information, the researchers examined the frequency of hospitalization for GBS before and after the introduction of Ontario’s universal influenza immunization program.
The investigators identified 2173 GBS hospital admissions between June 1,1991 and March 31, 2004, which is equivalent to about 170 new cases per year. There was no statistically significant increase in admissions for GBS after the initiation of the immunization program.
“The absolute risk of GBS associated with the influenza vaccine is very, very, small — about 1 or 2 in every 1 million GBS hospitalizations,” said Dr. Wilson.
Patients considering influenza immunization should be advised of the GBS risk. However, said Dr. Wilson, because the risk is so low it should not influence an individual’s decision to get a flu vaccine. Rather, the primary consideration should be the potential benefit of vaccination to an individual.
Does Campylobacter Infection Play A Role?
While a potential mechanism between the influenza vaccine and GBS has not been identified, Dr. Wilson said there has been some speculation that it could be due to Campylobacter infection, which can occasionally contaminate egg media in which the vaccine is developed.
Previous research has shown an independent association between Campylobacter infection and GBS. Furthermore, he said, a recent paper in Journal of the American Medical Association speculated that the declining incidence of GBS might be linked to a commensurate decline in Campylobacter infection in the general population.
In addition, he said, it has been hypothesized that the influenza antigen itself may be involved. However, he said, at this point this is only theory, but an area that may be one for future research.
In the meantime, Dr. Wilson, said adverse event reporting systems to track complications related to the influenza vaccine would be useful and allow investigators to determine whether risk varies from year to year.
Arch Intern Med. 2006;166:2217-2221.