CDC: GBS risk similar for H1N1 and seasonal flu vaccines

Jun 2, 2010 (CIDRAP News) – The risk for the paralytic condition called Guillain-Barre syndrome (GBS) in people who received pandemic H1N1 influenza vaccine was about the same as the risk among those who receive seasonal flu shots, according to preliminary findings released today by the US Centers for Disease Control and Prevention (CDC).

GBS is a rare side effect of illness or vaccination and was linked to the 1976 swine flu vaccine. Today's preliminary analysis from the CDC's Emerging Infections Program (EIP) suggests there were 0.8 excess cases of GBS per 1 million vaccinations, whereas surveillance for the 1976 pandemic vaccine showed about 10 excess cases per 1 million vaccinations. The CDC published the findings in an early-release edition of Morbidity and Mortality Weekly Report (MMWR).

The CDC's EIP, a collaboration between state health departments and academic centers, conducts disease surveillance in 10 states. Today's analysis covers patients who were hospitalized with GBS after September 30, 2009, just as the vaccine launched. Researchers interviewed patients who met the GBS classification definition by phone to gather more information about their medical and vaccination history.

The group calculated GBS incidence for vaccinated and unvaccinated populations by using data from the CDC's Behavioral Risk Factor Surveillance System and its national 2009 H1N1 flu telephone survey.

The EIP identified 326 GBS cases that met the case criteria, of which 27 patients had received the pandemic vaccine within 42 days before illness onset and 274 had not received the vaccine. Status was unknown or pending for 25 of the cases. Sixteen (59%) of the 27 who had received the pandemic vaccine also reported other illness symptoms in the 42 days before GBS onset. Meanwhile, 78% of unvaccinated patients reported other illness symptoms before GBS onset.

Investigators didn't see clustering of cases during certain time periods after pandemic vaccination, a pattern that was seen 2 and 3 weeks after 1976 swine flu vaccination and in one study of seasonal flu vaccine during two flu seasons in the early 1990s.

Death from any cause was noted in 8 (2%) of the 326 GBS patients, none of whom had received the pandemic vaccine within 42 days of illness onset.

The authors said the preliminary analysis, if confirmed, shows the excess GBS risk related to the pandemic vaccine is in line with the risk from seasonal flu vaccines and is much smaller than the risk linked to the 1976 swine flu vaccine.

They noted the high proportion of other illnesses, such as gastrointestinal conditions or respiratory infections, in the patients with GBS, which suggests that a number of GBS illnesses that were observed after vaccination could be linked to other causes. "Historically, 40% to 70% of GBS patients report experiencing an antecedent infectious illness," the article states.

EIP surveillance findings appear to be consistent with results from other pandemic vaccine safety monitoring systems run by the CDC, which have not detected a statistically significant association between GBS and pandemic H1N1 flu vaccination.

Among the report's limitations, the authors said some cases could have been misclassified and that vaccination status could have been inaccurately reported for some patients. They suggested that vaccination rate estimates could be overestimated by as much as two or three percentage points and said none of the vaccine monitoring systems can account for all potential confounding risk factors for GBS. "Thus, the association described above cannot prove a causal relationship between vaccination and GBS," the group wrote.

A final analysis of EIP GBS data should be available by early fall, according to today's report.

CDC. Preliminary results: surveillance for Guillain-Barre syndrome after receipt of influenza A H1N1 monovalent vaccine—United States, 2009-2010. MMWR 2010 Jun 2;59:1-5 [Full text]

See also:

Dec 4, 2009. CIDRAP News "CDC heartened by initial safety reports on H1N1 vaccine"

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