For every 10 million H1N1 vaccinations administered in the U.S. last year, officials received about six reports of people developing Guillain-Barré Syndrome, a rare neurological disorder.
It's a low number, and one that officials had predicted they would see, considering the fact researchers know the risk of developing the syndrome following vaccination is quite low.
But memories of problems during a 1976 immunization campaign and public anxiety during last year's H1N1 pandemic stoked fears that mass inoculation using a new vaccine would result in a rash of serious side effects, health problems and even deaths. As a result, public health officials in Canada and around the world have been closely monitoring reports of medical problems or other adverse events associated with those who were vaccinated.
Now that the peak of the pandemic has passed and researchers are looking back at the response, signs indicate many of those public fears were exaggerated, or even unfounded.
U.S. researchers presented a study yesterday at the American Academy of Neurology's annual meeting, taking place in Toronto this week, that showed reports of Guillain-Barré Syndrome associated with H1N1 vaccination in the U.S. were extremely low.
Specifically, there were 6.2 reports of the syndrome for every 10 million H1N1 vaccinations administered last year. Meanwhile, researchers said there were 10.6 reports of GBS for every 10 million seasonal flu vaccinations given last year. The rates are low, and similar to those observed during other vaccination campaigns.
It's one of the first major reports on the subject to be released following the pandemic. The Public Health Agency of Canada said there have been 26 cases of GBS reported in Canada following the H1N1 vaccination campaign, which equals about one case for every million doses of vaccine distributed.
Guillain-Barré Syndrome is a rare disorder in which people experience sudden weakness or paralysis. The syndrome is more common in people over age 50, and most people who develop it are able to recover. Although the exact causes are somewhat murky, the Public Health Agency of Canada says strong evidence has linked it to Campylobacter jejuni, a type of food-borne bacteria. There is evidence that vaccinations are linked to a slight increase in the cases of GBS, although the reasons are unclear. The U.S. Centers for Disease Control and Prevention says it seems the immune systems of people with GBS damage their own nerve cells. This immune system response may be activated by certain infections, and very rarely, vaccines, according to the agency.
There are about 600 to 700 new cases of GBS in Canada each year, says the Public Health Agency of Canada. Before last year's pandemic, about 10 to 12 million flu vaccines were distributed across Canada each year. But only 79 cases of GBS following vaccination have been recorded in Canada since 1997, which is about seven cases a year.
While the new report provides some reassurance of the vaccine's safety, it also raises a number of new questions. For instance, why are reports of GBS higher for the seasonal flu vaccine? How exactly are vaccines linked to Guillain-Barré Syndrome? And perhaps most important, who is most at risk of developing the rare disorder?
James Sejvar, a neurologist and epidemiologist at the U.S. Centers for Disease Control and Prevention, told reporters that work must be done to determine whether there is a genetic risk that makes some people susceptible to the syndrome. He also said the issue is complicated by the fact that it's difficult to know whether any cases of GBS can be linked to last year's vaccination campaign.
Dr. Sejvar said he couldn't speculate on why the syndrome seemed higher among those who had the seasonal vaccine. But he said the CDC is doing several large studies that are actively following people who get vaccinated in order to more accurately track the rate of side effects.
Nizar Souayah, assistant professor of neuromuscular disease at the New Jersey Medical School in Newark, who authored the study, said it's difficult to draw many conclusions from the research because the issue is extremely complex and is still unfolding. He said he wants to embark on a new study to track people following vaccination to determine whether they developed any side effects.