Small, Increased Risk of GBS With Pandemic H1N1 Vaccine
Preliminary data from the CDC’s Emerging Infections Program* (EIP) suggest a small, but statistically significant, increased risk of Guillain-Barre syndrome (GBS) with receipt of the 2009 pandemic H1N1 vaccine. However, the risk, 0.8 excess cases per 1 million vaccinations, is comparable to that seen with other seasonal flu vaccines and is much lower than the observed risk of GBS during the 1976 swine-flu vaccination program (~10 excess cases per 1 million vaccinations). The excess risk of GBS with the 2009 pandemic H1N1 vaccine is also much lower than the risk of hospitalization or death due to illness with the virus itself (222 and 9.7 per 1 million).
Within EIP’s surveillance area, there were 326 cases of GBS (meeting case criteria) between October 1, 2009, and May 10, 2010. Among these patients, 27 received documented vaccination against the 2009 pandemic H1N1 virus during the 42 days preceding their illness, and 274 did not (the vaccine status of 25 was unknown or “pending ascertainment”). Consequently the estimated age-adjusted rates of GBS were 1.92 and 1.21 per 100,000 person-years among vaccinated and unvaccinated persons, respectively. These data led to an attributable excess risk of GBS with vaccination of 0.8 per 1 million vaccinations.
Once again, it’s important to remember that association does not mean causation. For instance, many of the vaccinated persons (59%) also reported illness symptoms during the 42 days before GBS onset, which is consistent with many cases of non-vaccine-related cases of GBS. Vaccination, in these cases, could have been completely incidental to the development of neuropathy. Moreover, there was no temporal clustering of GBS onset after vaccination (eg, at 2 weeks), suggesting that vaccine was not causally related to illness. Vaccinated persons with GBS were not sicker, nor did they suffer worse outcomes, than unvaccinated individuals with GBS.
The CDC advises that the final assessment of the EIP data will be available in the fall of this year. Other safety databases, like the Vaccine Safety Datalink, have not shown an association between GBS and receipt of the 2009 pandemic H1N1 vaccine.
* A collaboration among the CDC, state health departments, and academic centers in 10 states. Surveillance covers about 45 million individuals.