There are long-standing concerns that a small number of people develop a paralytic illness after getting a flu shot. But a new study shows that the risk of developing the condition, Guillain-Barré syndrome, is actually 16 times higher after contracting influenza than after vaccination.
In the general population, about one in 100,000 people develop GBS, a baffling and potentially fatal condition that resembles polio.
The new findings, published in the medical journal The Lancet Infectious Diseases, show that one additional case of GBS occurs after every million people who get a flu shot. But an additional case of GBS springs up in every 60,000 people who are treated for the flu.
Dr. Jeff Kwong, a scientist at the Toronto-based Institute for Clinical Evaluative Sciences and lead author of the research, said the findings should reassure people about the safety of influenza vaccination and remind them the flu itself carries many more health risks.
"The message here is that people shouldn't worry so much about GBS," he said. "We're actually comparing two very small risks, but the risk of not getting vaccinated is higher."
The study used data collected in Ontario between 1993 and 2011. They looked at patients who developed GBS either after a flu shot or following the flu. They calculated there were 1.03 GBS cases per million among the vaccinated group compared to 17.2 GBS cases per million patients treated for the flu.
Guillain-Barré Syndrome is a progressive but often reversible paralysis that begins in the feet and creeps upward. Most people make a full recovery, but a small percentage die.
GBS is caused by a malfunction of the immune system, usually triggered by an infection. For example, Campylobacter jejuni, a bacterium that causes gastroenteritis, can trigger the disease. So can infection with influenza virus and, to a lesser extent, the flu vaccine, which tricks the immune system into believing there is a threat so it will produce antibodies to fight the flu virus.
The link between GBS and flu shots has been well documented since 1976, when the fear of a pandemic strain of H1N1 swine flu prompted a mass vaccination campaign in the United States. What followed was a noticeable spike in GBS, with more than 500 cases and 25 deaths linked in people vaccinated.
When a new strain of H1N1 swine flu emerged, there were concerns this problem could re-emerge, so there was close monitoring. The Public Health Agency recorded 56 cases of GBS among Canadians who received the swine flu vaccine in 2009. There are about 700 cases of Guillain-Barré Syndrome in Canada each year, and only a small fraction are related to the flu shot.
Between 1997 and 2009, there were 79 cases of GBS linked to influenza vaccination, meaning seven annually. Those numbers shot up during the H1N1 campaign and have dropped back down again since.
Approximately 10 million Canadians get a flu shot each year, and another eight million or so contract influenza. The flu and its complications kill between 4,000 and 8,000 people a year in Canada, according to the Public Health Agency of Canada.