Swine flu vaccine and risk of Guillain-Barré Syndrome

By Dr. Ananya Mandal, MDJul 13 2011

In a large five nation study from Europe it was seen that the use of adjuvanted vaccines against the pandemic H1N1 influenza virus probably did not increase the risk of Guillain-Barré Syndrome (GBS), but the investigators could not firmly rule out a slightly greater chance of suffering the paralytic condition.

European researchers in the BMJ report that the adjusted odds ratio (OR) for GBS in those who were vaccinated was 1.0, indicating no higher risk, but the 95% confidence interval (CI) was 0.3 to 2.7, leaving the possibility open. The report reads, “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 up to 2.7-fold or three excess cases per one million vaccinated people.”

US swine flu vaccination campaign of 1976 first showed the association of GBS with flu vaccination. Then almost one extra GBS case was reported per 100,000 vaccinations. Since then, studies of seasonal flu campaigns have shown no increase or only a slightly increased risk of 1 to 2 extra cases per million vaccine takers, according to an editorial accompanying the BMJ report.

In Europe, the 2009 pandemic triggered the first widespread use of flu vaccines containing adjuvants: GlaxoSmithKline's Pandemrix and Novartis's Focetria, which were the most widely used vaccines there. Adjuvants have not been used in US flu vaccines.

This new study was conducted in Denmark, France, the Netherlands, Sweden, and the United Kingdom by the Vaccine Adverse Events Surveillance & Communication (VAESCO) consortium. The study period was November 2009 through March 2010. The researchers identified GBS cases, matched each patient with up to 25 controls by age, sex, onset date, and country, and sought to determine whether they had received an H1N1 vaccine within 6 weeks before the date of GBS onset.

Results revealed 154 GBS cases and managed to match 104 of them with one or more controls. Most of the case-patients were men between the ages of 46 and 61. Six of the patients required mechanical ventilation or died of the condition. Pandemrix was the most widely used pandemic vaccine in the study population, the report says.

The investigators found that the unadjusted ORs for GBS ranged from 1.3 to 2.5 in the UK, Sweden, and the Netherlands. In Denmark the adjusted OR was 9.5 on the basis of just two cases, while the risk for France could not be estimated because there was only one vaccinated case-patient and no vaccinated controls. However, the apparent increase in risk largely disappeared when the researchers adjusted for flu-like illness or upper respiratory tract infection and seasonal flu vaccination in their subjects. Those adjustments yielded the OR of 1.0 (95% CI, 0.3 to 2.7).

The authors say that with the “apparent strong confounding” effects of flu-like illness and seasonal flu vaccination, they could find no increase in GBS risk in a source population of around 50 million people in Europe.

The researchers say they are working on a larger study that will include cases from additional countries and cover a longer period.

In the accompanying editorial, four specialists with the US Centers for Disease Control and Prevention (CDC) describe the findings as being in accord with studies on the GBS risk associated with non-adjuvanted pandemic H1N1 vaccines but as puzzling in some respects. The editorial reads, “Overall, the results suggest that if there was an increased risk associated with the adjuvanted 2009 H1N1 vaccines studied, it was considerably smaller than that seen with the 1976 flu vaccines. Whether there was an increased risk, however, is not clear.”