July 14, 2011 — The risk for Guillain-Barré syndrome is not increased after use of adjuvanted pandemic influenza (H1N1) 2009 vaccine, but the upper limit does not exclude a potential increase in risk up to 2.7-fold, according to the results of a multinational case-control study reported online July 12 in the BMJ.
"A concern with the pandemic influenza A (H1N1) 2009 vaccine was the possible occurrence of neuroimmunological adverse events, including Guillain-Barré syndrome," write Jeanne Dieleman, senior pharmacoepidemiologist from the Department of Medical Informatics at Erasmus University Medical Center in Rotterdam, the Netherlands, and colleagues. "A more than sevenfold increased risk of Guillain-Barré syndrome was observed with the swine origin influenza A (H1N1) subtype A/NJ/76 vaccine applied in the United States in 1976, when the vaccination campaign had to be discontinued abruptly. Subsequent prospective surveillance studies and retrospective epidemiological studies on seasonal influenza vaccines used in 1978, 1979, 1980, 1992, 1993, and beyond showed no or modest increases (up to twofold) in risk of Guillain-Barré syndrome."
The goal of this case-control study, performed in 5 European countries, was to evaluate the relationship between pandemic influenza A (H1N1) 2009 vaccine and Guillain-Barré syndrome. Patients (n = 104) with Guillain-Barré syndrome or its variant Miller-Fisher syndrome were classified according to the Brighton Collaboration definition and matched to 1 or more control participants by age, sex, index date, and country. The primary study endpoint was the relative risk estimate for Guillain-Barré syndrome after vaccination with pandemic influenza vaccine.
Countries varied substantially in case recruitment and vaccine coverage. Adjuvanted vaccines (Pandemrix and Focetria) were used most commonly. For all countries, the unadjusted pooled risk estimate was 2.8 (95% confidence interval [CI], 1.3 - 6.0). However, vaccination with pandemic influenza vaccine was not associated with a heightened risk for Guillain-Barré syndrome after adjustment for influenza-like illness or upper respiratory tract infection and seasonal influenza vaccination (adjusted odds ratio [OR], 1.0; 95% CI, 0.3 - 2.7). On the basis of the 95% CI, the absolute effect of vaccination could vary from 1 less case of Guillain-Barré syndrome within 6 weeks after vaccination in 1 million people, to up to 3 excess cases.
"The risk of occurrence of Guillain-Barré syndrome is not increased after pandemic influenza vaccine, although the upper limit does not exclude a potential increase in risk up to 2.7-fold or three excess cases per one million vaccinated people," the study authors write. "When assessing the association between pandemic influenza vaccines and Guillain-Barré syndrome it is important to account for the effects of influenza-like illness/upper respiratory tract infection, seasonal influenza vaccination, and calendar time."
Study limitations include the fact that the study was performed in a pandemic situation; under-reporting of cases in the Netherlands; substantial delays in inclusion of cases in Sweden and France; possible residual confounding; and some missing information on vaccination, mostly in control participants.
Editorial: Safety Implications of H1N1 2009 Vaccines
In an accompanying editorial, Frank DeStefano, from the Immunization Safety Office (MS-D26), Division of Healthcare Quality Promotion, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention in Atlanta, Georgia, and colleagues discuss the safety of adjuvanted pandemic influenza A (H1N1) 2009 vaccines.
"Although pandemic influenza A (H1N1) 2009 monovalent vaccines are no longer being used, data on their safety are relevant to current clinical practice because the H1N1 strain in the pandemic vaccine has been incorporated into the currently recommended trivalent seasonal vaccine," the editorialists write. "Most of the trivalent seasonal flu vaccines currently used in Europe do not contain an adjuvant, and no adjuvanted flu vaccines are used in the US. Nonetheless, the safety findings on adjuvanted flu vaccines will be important if such vaccines become more common in the future, whether in seasonal flu vaccines or for the next pandemic."
This study was funded by the European Centre for Disease Prevention and Control. Some of the study authors have disclosed various financial relationships with GSK, SPMSD, Wyeth/Pfizer, Boehringer, Lilly, AstraZeneca, and/or Baxter. The editorialists have disclosed no relevant financial relationships.
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