Americans who received the H1NI flu vaccine, also known as the swine flu vaccine, in 2009 and 2010 had a very small but real increase in their risk for developing a rare neurological disorder, a new study finds.
Among people who were vaccinated, cases of Guillain-Barré syndrome were about twice as common during the first six weeks after receiving the vaccination than they were after that.
However, because a large number of people received the shot, and because Guillain-Barré syndrome would be expected to occur in some individuals regardless of whether or not they had been vaccinated, the actual increase in risk was extremely small: about 1.5 extra cases of the condition for every 1 million people vaccinated, the researchers said.
Given that the swine flu vaccine is thought to have prevented 700,000 to 1.5 million cases of flu, and 4,000 to 10,000 hospital admissions during the years in which it was given, the benefits of the vaccine greatly outweighed the risks, the researchers said.
Guillain-Barré syndrome occurs when the immune system attacks nerve cells, causing weakness and tingling in the legs and arms, and sometimes, full body paralysis, according to the National Institutes of Health. The swine flu vaccine used during the 1976 swine flu outbreak was linked to an increased risk of Guillain-Barré syndrome of about 1 additional case for every 100,000 people vaccinated. However, since then, the seasonal flu vaccine has not been linked to an increased risk of Guillain-Barré syndrome, possibly because the amount of data collected from an individual flu season is too small to find the link.
The magnitude of the 2009 vaccination program presented an opportunity for researchers to clarify the risk, and "demonstrate how low it actually is," said study researcher Dr. Bruce Gellin, director of the National Vaccine Program Office at the U.S. Department of Health & Human Services.
The study analyzed information from 23 million people in the United States who received the 2009 H1N1 vaccine. Within six weeks of receiving the vaccine, 54 cases of Guillain-Barré syndrome were reported, compared with 29 cases during weeks seven to 13 after vaccination. The natural rate of Guillain-Barré syndrome in the U.S. population is 1 in 100,000.
It's not clear how the H1N1 vaccine could increase a person's risk of developing the syndrome. However, infections, including respiratory, stomach and even flu, have been linked to Guillain-Barré.
The study only found an association, and cannot prove that the swine flu vaccine causes Guillain-Barré syndrome. It's possible that Guillain-Barré syndrome is more common when swine flu vaccination peaks, which could affect the results, the researchers said.
The study will be published March 13 in the journal The Lancet.