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H1N1 (Swine Flu): Surveillance for Guillain-Barré Syndrome After Receipt of H1N1 Pandemic 2009 Monovalent Vaccine in the U.S.

June 3, 2010

The CDC has just issued a preliminary report in their weekly publication, the Morbidity and Mortality Weekly Report (MMWR), on the occurrence of Guillain-Barré syndrome (GBS) following vaccination with the H1N1 pandemic vaccine.

GBS is an uncommon peripheral neuropathy causing paralysis and in severe cases respiratory failure and death. GBS often follows an antecedent gastrointestinal or upper respiratory illness but, in rare cases, can follow vaccination. In 1976, vaccination against the novel swine-origin influenza A virus was associated with a statistically significant increased risk for GBS in the 42 days after vaccination (approximately 10 excess cases per 1 million vaccinations), a consideration in halting the vaccination program in the context of limited influenza virus transmission.

To monitor the safety of the H1N1 pandemic vaccine, several federal surveillance systems, including CDC’s Emerging Infections Program (EIP), are being used. In October 2009, EIP began active surveillance to assess the risk for GBS after 2009 H1N1 vaccination. Preliminary results from an analysis in EIP comparing GBS patients hospitalized through March 31, 2010, who did and did not receive 2009 H1N1 vaccination showed an estimated age-adjusted rate ratio of the following:

  • 1.77 (GBS incidence of 1.92 per 100,000 person-years among vaccinated persons compared to 1.21 per 100,000 person-years among unvaccinated persons).
  • If end-of-surveillance analysis confirms this finding, this would correspond to 0.8 excess cases of GBS per 1 million vaccinations, similar to that found in seasonal influenza vaccines.

No other federal system to date has detected a statistically significant association between GBS and 2009 H1N1 vaccination. Surveillance and further analysis is ongoing. The 2009 H1N1 vaccine safety profile is similar to that for seasonal influenza vaccines, which have an excellent safety record. Vaccination remains the most effective method to prevent serious illness and death from 2009 H1N1 influenza infection; illness from the 2009 H1N1 influenza virus has been associated with a hospitalization rate of 222 per 1 million and a death rate of 9.7 per 1 million population.

In addition to existing surveillance systems that routinely monitor vaccine safety in U.S. vaccine recipients, new systems were added in the fall of 2009.  The 2009–10 influenza vaccine safety network consists of the following systems:

  • Vaccine Adverse Event Reporting System (VAERS)
  • Real Time Immunization Monitoring Systems (RTIMS)
  • Vaccine Safety Datalink (VSD)
  • Department of Defense (DoD) Defense Medical Surveillance System (DMSS)
  • Post-Licensure Rapid Immunization Safety Monitoring (PRISM)
  • Indian Health Service (IHS), Department of Veteran Affairs (VA)
  • Centers for Medicaid and Medicare Services (CMS)
  • CDC’s EIP

This MMWR discusses preliminary analyses from the EIP, which is a collaboration between the CDC, state health departments, and academic centers in 10 states.  EIP initiated a population-based, active surveillance program designed to provide rapid case identification and assessment of risk for GBS following 2009 H1N1 vaccination.  GBS incidence was calculated and compared for the vaccinated and unvaccinated populations, which were estimated by age group, using data from CDC’s Behavioral Risk Factor Surveillance System (BRFSS) and National 2009 H1N1 Flu Survey (NHFS) telephone survey data for the counties in the EIP catchment areas, using methods published previously.

During October 1, 2009–May 10, 2010, a total of 529 reports of potential GBS were identified by EIP, of which 326 met the GBS case criteria.  Of the 326 persons with GBS:

  • 27 had documentation of 2009 H1N1 vaccination in the 42 days preceding illness onset.
  • 274 did not receive vaccine, and the vaccine status of 25 was either unknown (six) or pending ascertainment (19)
  • 16 of the 27 (59%) with documentation of 2009 H1N1 vaccination also reported antecedent illness symptoms in the 42 days before GBS onset
  • 78% of unvaccinated persons with GBS (215 of 274) reported antecedent symptoms (p=0.04).
  • No clustering among vaccinated persons was observed in the period between vaccination and illness onset (p=0.54).
  • Among the 27 GBS patients with 2009 H1N1 vaccination, four required ventilator support, and one remained hospitalized 30 days after GBS onset
  • Among the 274 GBS patients who did not receive 2009 H1N1 vaccination, 37 (14%) required ventilator support, and 34 (12%) remained hospitalized after 30 days.
  • Eight (2%) of the 326 GBS patients died (from any cause)
  • None of the eight had received the 2009 H1N1 vaccine within 42 days of illness onset.

CDC Editorial Note

This preliminary analysis showed an elevated, statistically significant association between 2009 H1N1 vaccination and GBS. If confirmed, the excess risk for GBS associated with 2009 H1N1 vaccine of 0.8 cases per 1 million vaccinations would be comparable to the excess described previously for some trivalent seasonal influenza vaccine formulations (approximately one excess case per 1 million vaccinations), and much smaller than the risk for GBS observed during the 1976 swine influenza vaccine campaign (approximately 10 excess cases per 1 million vaccinations)

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