H1N1 (Swine Flu) and Guillain-Barré Paralysis

Risk Needs Realistic Evaluation

Sep 16, 2009 James Cooper

The association, if any, needs to be considered in light of absolute risk, not relative risk.

“Report New Cases of Guillain-Barré After H1N1 Flu Vaccination,” the headline declared. But the story is only precautionary. Neurologists are asked to be alert to the possible association of H1N1 (swine) flu vaccination and new cases of Guillain-Barré syndrome (GBS). The story did not say there was an association; neurologists, who are likely to see patients with GBS, were urged to inquire about previous vaccinations in case an association does exist.

What is Guillain-Barré Syndrome?

GBS is a condition that affects peripheral nerves, the nerves from the brain or spinal column to muscles. It makes the muscles weak or paralyzes them. It is sometimes called Ascending Paralysis because paralysis often starts in the legs, then works its way up, and may affect the breathing muscles. GBS is in the group of diseases called “acute flaccid paralysis,” which includes enteroviral neuropathy, transverse myelitis, traumatic neuritis, and other nervous system disorders.

The condition may last days to months. Deaths are quite unusual.

GBS is very rare. Sometimes it has been associated with gastrointestinal illness, and in 1976, it was associated with flu vaccination in the US.

Association of GBS and Flu Vaccination

An unsolved mystery is why there was an increased risk of GBS after swine flu vaccinations in 1976-1977. That mystery led to much public and scientific debate, and the National Influenza Immunization Program was suspended on December 16, 1976. It also led to establishment of more stringent reporting systems. In 1990, the Vaccine Adverse Events Reporting System was established, and the picture of vaccination safety (or lack of it) is now much more clear.

GBS and Vaccination Data

Background Incidence: In the absence of vaccination, GBS occurs in about one or two people per 100,000 per year. (Schoenberg, BS. Advances Neurology 1978;19:249-60.)

In 1976-1977: among those vaccinated, there appeared to be just under one additional case. This was the attributable risk, or excess risk. Note that the relative risk is about double the background risk—one could say, “Vaccination doubled the risk of GBS!” Still, the absolute risk, maybe three cases per 100,000, would be very low.

In 1978-1979: “In contrast to (1976-1977) the 1978-1979 influenza vaccine was not associated with a statistically significant excess risk of GBS.” (Hurwitz ES, et al.New England Journal of Medicine. 1981 Jun 25;304(26):1557-61)

1992-1993 to 1993-1994: One additional case of GBS per million vaccinated persons was reported (one tenth case per 100,000).

From 1990 through June 2003: The rate of GBS among people vaccinated decreased to 0.04 per 100,000 vaccinees. (Haber P, et al. JAMA. 2004 Nov 24;292(20):2478-81)

Absolute Risk vs. Attributable or Excess Risk

Using the latest figures, a back of the envelope calculation looks something like this:

Without vaccination, one’s risk of GBS is one or two per 100,000 people.
Vaccination may (or may not) increase the risk by less than a tenth of one case per 100,000. The risk would be something like one in two million.
The risk of death from H1N1 influenza (the disease) is unknown at this time; it may be as low as one percent, or as high as ten percent. If it is one percent, the risk of death is one thousand per 100,000 cases.

Most public health experts say the risk of vaccination is very low, while the risk of H1N1 swine flu is significantly higher.

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Image of the H1N1 Virus