H1N1 Vaccines Linked to Guillain-Barre Syndrome but Not Birth Defects

Two new studies confirm that the benefits of getting a flu shot outweigh the very small risks

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How safe are flu vaccines? Two new studies show that the H1N1 vaccine poses no risk of birth defects when given to pregnant women, but does slightly increase the risk of Guillain-Barre syndrome, a reversible autoimmune disorder, in patients over 50.

In the studies, published in the Journal of the American Medical Association, scientists tracked the rates of birth defects and developmental problems such as low birth weight and preterm birth in babies born to Danish women who were vaccinated against H1N1 in 2009-10 during pregnancy. A separate group of scientists in Quebec looked at the risk of Guillain-Barre syndrome in millions of people given the vaccine in Canada, also in 2009-10.

In the study that focused on Guillain-Barre, Philippe De Wals, a professor of social and preventive medicine at Laval University in Canada, and his colleagues included 4.4 million people in Quebec who received the H1N1 vaccine as part of a mass immunization program during the 2009 “swine flu” pandemic; they were all vaccinated over a six-month period from October 2009 to March 2010.

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Researchers tracked rates of Guillain-Barre infection for up to eight weeks following vaccination, and found a very small increase in risk in vaccinated people: overall, there were 83 confirmed cases of Guillain-Barre, including 25 among people who had received the H1N1 vaccine. The risk of developing the syndrome, in which the body’s own immune system attacks the nerves, was doubled among vaccinated people, compared with the general population. But the absolute risk was small: based on the data, the authors calculated that about 2 people would be expected to develop Guillain-Barre per every million who were vaccinated — a risk that was limited to adults over 50.

“Doubling of a very low risk is still a very low risk,” says De Wals. “The one or two chance per million doses of getting Guillain-Barre is much lower than the chances of getting influenza, being hospitalized and dying of the flu.”

The connection between H1N1 vaccines and Guillain-Barre first emerged during the swine flu pandemic in 1976, when public health experts ordered mass immunizations against an H1N1 influenza virus related to the one seen in 2009; health officials saw slight increases in the syndrome following the mass inoculations.

After reviewing data on the link, the Institute of Medicine concluded that the vaccine was responsible for the increased risk of Guillain-Barre, and that some people were more susceptible to flu-shot-triggered cases than others. Scientists said that by producing antibodies against the flu virus in response to the vaccine, some people may inadvertently generate antibodies against their own cells as well, causing the disorder.


De Wals and his colleagues did not investigate whether conventional seasonal flu vaccines could carry a similar risk, but because all immunizations against flu work the same way, it’s a possibility, says De Wals. The reason that Guillain-Barre risk emerges specifically with H1N1 vaccines is likely because it has been used more widely; H1N1 spreads into a pandemic more quickly than other flu strains, which means that more people get vaccinated against H1N1 than against seasonal flu (generally only about 30% of people get their flu shots every year.)


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In the study involving H1N1 vaccination in pregnant women, researchers came up with good news: the vaccine appears to pose no risk to unborn babies. Researchers tracked 53,432 infants born between November 2009 and September 2010 in Denmark and found no statistically significant difference in the rates of birth defects, preterm birth or low birth weight between babies whose mothers were vaccinated and those whose mothers were not. About 5.5% of babies whose mothers got the flu shot were born with birth defects, compared with 4.5% of those whose mother did not get immunized; 9.4% of infants exposed to the flu shot in utero were born prematurely, compared with 7.3% of those who weren’t exposed.

“This study is important because the data are absolutely reassuring,” says Dr. Michael Katz, interim medical director and senior adviser for the March of Dimes, who was not involved in the study. “If a woman gets influenza while pregnant, it puts the fetus at risk. So protecting her with vaccination is important.”


The H1N1 shot used in the 2009 outbreak was especially worrisome to pregnant women, because it was a new vaccine developed in quick response to the pandemic flu. While safety studies were conducted, Katz says that any time a new vaccine is introduced to pregnant women, there is concern for how it might affect the unborn baby.

But the benefits of immunizing moms-to-be against influenza, which can have devastating health consequences for developing fetuses, outweigh the potential harms. And the results of the new study, while not the last word on H1N1 vaccines in pregnancy, should be comforting to women and doctors.

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Similarly, De Wals says the small risk of Guillain-Barre is not significant enough to argue against the benefits of flu vaccination. He recognizes that the findings may bolster some antivaccine advocates’ arguments, but says the “small risk is acceptable in terms of the balance of risk, considering that influenza is not always a benign disease.”