Any speculation drawing an ongoing link between flu vaccination and the risk of a rare, paralyzing neuromuscular disorder has been dashed by a huge study. An analysis of side effects recorded among nearly 90 million people in China who were vaccinated during the 2009–2010 flu season found that only 11 people subsequently were diagnosed with Guillain-Barré syndrome, a rate no greater than what normally appears in the population. The study appears online February 2 in the New England Journal of Medicine.

In 1976, a strain of swine flu showed up in the United States, prompting the manufacture and delivery more than 40 million doses of vaccine against it. The epidemic ultimately never materialized, but studies noted that hundreds of cases of Guillain-Barré syndrome were reported after the vaccination campaign. The vaccine was withdrawn. In 2003, an Institute of Medicine review found that the evidence pointed to an association between the 1976 swine flu vaccine and the syndrome.

IOM found no clear evidence of such a link with subsequent flu vaccines, but some concerns have lingered vis-à-vis flu vaccination.

These fears intensified in 2009 when another swine flu emerged, this time known as the H1N1 flu, and a vaccine was made for it. After mass vaccinations, physician Yu Wang and colleagues at the Chinese Centers for Disease Control and Prevention in Beijing collected data on all adverse effects reported by the 89.6 million people in China who received the flu vaccine in 2009 and 2010. The researchers found an exceptionally low rate of Guillain-Barré syndrome among those who had been vaccinated — less than the background rate in the population.

“This was a generally well-done study given the limitations of a voluntary reporting system,” says Penina Haber, an epidemiologist at the U.S. CDC. The results are similar to a U.S. CDC analysis of adverse events for vaccines and provide further evidence of the safety of the 2009 H1N1 influenza vaccine, she says.

Grace Lee, a physician and epidemiologist at Harvard Medical School and Harvard Pilgrim Health Care Institute in Boston, notes that the actual rate of Guillain-Barré syndrome reflected in the Chinese study was based on reporting from a wide network of health officials. “It’s hard to be sure the actual rate of events is accurate,” she says. But she says that the size of the database probably makes up for any weaknesses in data collection. “It’s a great study because this is such a huge population. The utility of this passive surveillance system is that there is sheer power in the numbers to pick up unanticipated adverse events.”

Image: Flickr/ghinson.

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