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13-10-2011 | General practice | Article

Neurologic and immunologic side effects uncommon with H1N1 vaccine


Free abstract

MedWire News: The pandemic influenza A (H1N1) vaccine Pandemrix is not associated with increased risks for Guillain-Barré syndrome, multiple sclerosis, type 1 diabetes, or rheumatoid arthritis, Swedish study data show.

However, vaccination was associated with an increased risk for Bell's palsy, paresthesia, and inflammatory bowel disease in individuals with comorbidities.

Pandemrix (GlaxoSmithKline, Middlesex, UK) is an adjuvant-based vaccine that was authorized, along with two others, by the European Medicines Agency in September 2009, explain Carola Bardage (Medical Products Agency, Uppsala) and colleagues in the BMJ.

Adjuvants can reduce the amount of antigen needed to provide an adequate immunologic response and reinforce the ability to provide long-lasting protection. However, their autoimmune-stimulating potential and an increased risk for neurological adverse events, such as the Guillain-Barré syndrome, are safety concerns.

In the present study, Bardage and team examined the risk for the development of neurologic and autoimmune disorders in people vaccinated against H1N1 with Pandemrix, compared with unvaccinated people, over an 8-10 month period.

The study population comprised all 1.98 million people registered on 1st October 2009 as having lived in Stockholm County since 1st January 1998. Of these, 1,024,019 (52.6%) received the H1N1 vaccine between October 2009 and March 2010.

Overall, people who received the vaccine had a 25% increased risk for Bell's palsy and an 11% increased risk for paresthesia, compared with those who did not receive it, after adjustment for age, gender, socioeconomic status, and healthcare utilization (a surrogate marker of comorbidity). This corresponds to absolute excess risks in the vaccinated population of 8.4 cases per 100,000 vaccinated person years for Bell's palsy and 9.2 cases per 100,000 person years for paresthesia.

However, when the researchers examined overall risks in relation to the early and late phases of the vaccination campaign, they found that early vaccinations (≤45 days) - when high-risk groups predominated - were associated with a significantly increased risk for Bell's palsy (hazard ratio [HR]=1.34), paresthesia (HR=1.25), and inflammatory bowel disease (HR=1.25) in the 6 weeks post-vaccination, compared with no vaccination.

By contrast, risks for these conditions were not significantly increased in the subcohort of people vaccinated in the late phase of the campaign, who more closely represented the general population.

Risks for Guillain-Barré syndrome, multiple sclerosis, Type 1 diabetes, and rheumatoid arthritis were not increased in either subgroup.

Of note, only eight people aged 20 years and younger developed narcolepsy (six in the vaccinated cohort and two in the unvaccinated cohort), which has previously been linked to H1N1 vaccination in that age group. However, such small numbers "preclude any meaningful interpretation," says the team.

Bardage et al suggest that comorbidity could explain some of the excess risks seen in those individuals vaccinated early. The small excess risks "may be partly or entirely explained by other factors that were not captured by a crude measure of healthcare utilization," they write.

In spite of the risks observed, the authors conclude that their findings are "reassuring," particularly as there was no increase in the incidence of Guillain-Barré syndrome associated with H1N1 vaccination.

By Laura Dean

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