H1N1 flu vaccination associated with small excess risk of acquiring Guillain-Barré syndrome, but benefits outweigh risks

A meta-analysis of safety data gathered during the 2009 H1N1 vaccination programme in the US has established that the vaccination was associated with a small excess risk – about 1.6 extra cases per one million people vaccinated – of acquiring Guillain-Barré syndrome, a disorder of the nervous system which can result in temporary or more long-lasting paralysis and infrequently death.  The authors conclude that the benefits of the 2009 H1N1 vaccination campaign greatly outweighed the risks.

The research, published Online First in The Lancet, aimed to establish whether the Influenza A (H1N1) monovalent inactivated vaccine – which was studied among 23 million vaccinated people in the USA in 2009 in the largest mass vaccination effort in recent US history – was associated with an increased risk of Guillain-Barré syndrome, an autoimmune disorder of the peripheral nervous system, where the body starts to attack nerves involved in movement, and sometimes respiration and other functions.  Guillain-Barré syndrome is usually preceded by a viral or bacterial infection, and while it is a serious condition which patients typically take months to recover from, around 80% of patients experience a full recovery with appropriate treatment.

Using data from six adverse event monitoring systems which monitored vaccine safety during the 2009 vaccination programme, a group of researchers led by Dr Daniel Salmon, of the National Vaccine Program Office, US Department of Health and Human Services, analysed the number of cases of Guillain-Barré syndrome which occurred in about 23 million people who received the vaccine. 

In total, the researchers recorded 77 cases of Guillain-Barré syndrome occurring up to 91 days after administration of the H1N1 vaccine.  The researchers calculated that the incidence rate of Guillain-Barré syndrome was 2.35 times higher in the 42 days after vaccination compared with a later time period, and given that Guillain-Barré syndrome is thought to afflict about one person in 100,000 in the background population, this amounts to around 1.6 excess cases of the syndrome in every one million people vaccinated.

According to Dr Salmon, “On an individual level, we cannot predict with certainty who will contract influenza, who will have a serious complication or die from the disease, or who will have a very rare but serious adverse event from the vaccine. The safety monitoring programme for influenza A (H1N1) 2009 monovalent inactivated vaccine did not identify any other serious adverse events associated with the vaccine.” 

“About 61 million cases of influenza A (H1N1) disease were reported in the USA during the 2009 pandemic, including about 274 000 H1N1-related admissions and about 12 470 deaths. H1N1 vaccines offered substantial protection against medically attended illness. A recent study estimated that the H1N1 vaccination programme prevented 700 000–1 500 000 clinical cases of influenza, 4 000–10 000 admissions, and more than 200–500 deaths. Clinicians, policy makers, and those eligible for vaccination must consider the overall risks and benefits of vaccination, as defined by epidemiological studies, but should be assured that the benefits of influenza A (H1N1) 2009 monovalent inactivated vaccines greatly outweighed the risks.”

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