NEW YORK (Reuters Health) - In a review of data covering 13 years and millions of patients, researchers found no evidence of a link between being vaccinated against tetanus, hepatitis, pneumonia or flu, and developing the nerve-degenerating disorder Guillain-Barré.
"The take home message is vaccines are not causing Guillain-Barré Syndrome at a rate, if at all, that would possibly make the benefits of vaccination not worthwhile," wrote Dr. Daniel Salmon, of the Institute for Vaccine Safety at Johns Hopkins University, who was not part of the study, in an email to Reuters Health.
Guillain-Barré syndrome (GBS) is a rare condition that affects one person out of every 100,000 and can lead to paralysis, which is usually temporary.
GBS is considered an autoimmune response, in which a person's own immune cells attack the protective coating on nerve fibers. Most cases follow a bacterial or viral infection, and develop over the course of days or weeks.
In 1976, a vaccine created to protect against an epidemic of swine flu that never materialized was linked to an increased risk of GBS in people who got the shot. Ever since, researchers have been looking at whether flu vaccines or any other vaccines might be associated with heightened risk.
"There's definitely a connection in people's minds that vaccines cause this syndrome. But if you look at the (medical) literature, that doesn't bear out," said Dr. Roger Baxter, the new study's lead author and co-director of the Vaccine Study Center at Kaiser Permanente in Oakland, California.
Most studies have found no link between Guillain-Barré and vaccines, while a handful have shown a very small increase in risk among people who received specific flu vaccines. That includes a one in one million chance of GBS among older people who got the 2009 shot against the new H1N1 flu strain. (See Reuters Health story of July 10, 2012 here: reut.rs/NjEdg2)
Because the disorder is so rare, it's extremely difficult to determine whether a particular vaccine could have caused increases in cases, Baxter said.
So he and his colleagues looked to the large dataset of hospitalization records at Kaiser Permanente Northern California to see if they could spot a connection.
From 1995 to 2006, there were 415 cases of Guillain-Barré.
This was out of nearly 33 million person-years, a number that reflects both the number of people tracked and how long they were followed. For instance, 3.3 million people tracked for 10 years would represent 33 million person-years.
Among the 415 GBS cases, Baxter's team found, two-thirds had a documented gastrointestinal or respiratory infection in the weeks before developing GBS.
Just 25 people had received a vaccine of any kind within six weeks of the onset of GBS. Eighteen had flu shots, two got pneumonia vaccines, three had tetanus shots and three got hepatitis vaccines.
Baxter, who has received research grants from numerous vaccine manufacturers, said these numbers were no higher than expected.
"The bottom line is we think vaccines are very safe for this outcome, that they do not result in GBS, and if they do, it's so rare it's nothing to be worried about," he told Reuters Health.
The researchers also noted a seasonal pattern in the records, with GBS cases about 50 percent more common in winter. That result also agrees with prior research.
It's thought that seasonality in the appearance of Guillain-Barré cases may mirror seasonal rates of infections, especially with flu and other respiratory illnesses.
One recent French study found, for instance, that people who developed GBS were twice as likely to have had the flu or to have taken flu medication in the two months before the disorder set in.
Dr. Edward Belongia, director of the Epidemiology Research Center at Marshfield Clinic Research Foundation in Marshfield, Wisconsin, said it's important that studies like the current one monitor vaccines once they are on the market to spot possible safety concerns.
"It did happen in 1976, so we know it can happen," Belongia said.
Since then, however, "I think the evidence is clear, and this study just provides additional reassurance that vaccines are very safe with regard to GBS," he added.
SOURCE: bit.ly/11ayo8T Clinical Infectious Diseases, online April 11, 2013.