Vaccines are generally considered to be good news; they eradicated smallpox, could someday rid the earth of polio, and have made diseases such as whooping cough and measles into rare events instead of milestones of childhood. But they aren't without risks. A group of researchers looked at the hepatitis B vaccine and the risk of multiple sclerosis.

What the researchers wanted to know: Does getting vaccinated for hepatitis B increase your risk of multiple sclerosis?

What they did: The researchers from Harvard, Boston University, and the University of California, Irvine used the General Practice Research Database, which has anonymous medical information on over 3 million British people, supplied by their doctors. They picked out everyone who'd been diagnosed with MS between 1993 and 2000 and met certain criteria. Each person with MS was matched with as many as 10 controls - people without MS who were the same sex, approximately the same age, and had the same doctor.

What they found: Vaccination approximately tripled the risk of multiple sclerosis. 6.7 percent of people with multiple sclerosis had gotten at least one hepatitis B shot in the three years before their diagnosis, compared to just 2.4 percent of people who didn't have MS.

What the study means to you: It seems possible that the hepatitis B vaccine can either cause MS or make it appear sooner than it would. The researchers point out that 93 percent of people with MS hadn't been vaccinated recently, so obviously the vaccine is not the sole cause. Also, any dangers of the vaccine have to be weighed against the risks of a hepatitis B infection, which can be chronic and even lethal.

Caveats: Several other studies have failed to find a link between MS and hepatitis B vaccination. The authors of an editorial published in the journal Neurology along with the article write that there may be unrecognized biases in the study—for example, at the time covered by this study, hepatitis B vaccination was targeted at people who were considered at high risk for the disease. So prostitutes, drug addicts, people with liver disease, and people who traveled to certain regions of the world were much more likely to be vaccinated than the general population, and that could somehow contribute to their risk for multiple sclerosis. Or maybe healthcare workers, who were also targeted for vaccination, would be more likely to notice neurological symptoms of MS than other people would.

Find out more: A collection of information on hepatitis B from the National Library of Medicine: www.nlm.nih.gov

The Mayo Clinic explains multiple sclerosis: www.mayoclinic.com

Read the article and the editorial: Hernán, M.A., Jick, S.S., Olek, M.J., and H. Jick. "Recombinant Hepatitis B Vaccine and the Risk of Multiple Sclerosis." Neurology. Sept. 14, 2004, Vol. 63, No. 5, pp. 838–842.

Naismith, R.T. and A.H. Cross. "Does the Hepatitis B Vaccine Cause Multiple Sclerosis?" Neurology. Sept. 14, 2004, Vol. 63, No. 5, pp. 772–773. (editorial)

Abstract online: www.neurology.org