I heard that ABC's "20/20" did a program on the hepatitis B vaccination. They showed kids who had gotten the hepatitis B vaccine and ended up paralyzed or in a "vegetative" state because they had a certain gene. No one is sure what this gene is, so you don't know if your child has it. Have you heard of this? Why don't doctors tell you this before they give this vaccine to your child? I live in Texas and next year it is required before children can start school.

H.K.

I did not see the show to which you are referring, but since the invention of the first vaccine for any illness, there have been concerns about vaccine safety. For example, the original rabies vaccines carried an unacceptable risk of a serious brain disease. The early polio vaccines carried a significant risk of causing polio. In fact, this year, experts began recommending using the injected polio vaccine rather than the oral form to totally avoid the risk of transmitting polio through a vaccine.

For a few years in the late 1960s, the measles vaccine was a "killed" preparation, rather than containing live, but weakened virus. This occasionally resulted in an unexpected, serious illness if the person actually acquired measles. The pertussis (whooping cough) vaccine has been modified several times due to concerns about its safety. Today, experts recommend that everyone receive the "acellular" vaccine, which leads to fewer neurologic side effects than the other type.

There have also been numerous concerns about the safety of the hepatitis B vaccine, especially because it is now given to most infants born in the United States. There have been reports of sudden infant death syndrome (SIDS), multiple sclerosis, and a variety of other neurologic illnesses, including acute disseminated encephalomyelitis (ADEM) -- a severe, acute disease of the brain that leads to confusion and even death. ADEM usually follows mild viral illnesses, but has occasionally been noted to follow vaccination, especially after the old rabies vaccines.

The U.S. Centers for Disease Control and Prevention (CDC) and other groups have conducted several research studies looking at the safety of the hepatitis B vaccine. There has been no conclusive evidence to suggest that this vaccine causes any serious neurologic side effects. However, unless a side effect was very common, it would be very difficult indeed to prove it.

Let's take the following purely hypothetical statistics as examples. Let's say that ADEM naturally occurs in four in 1 million people per year, and that there are 200 million people in the country. (These numbers are made up for the sake of discussion.) We could then expect 800 cases of ADEM each year. Now let's say that the hepatitis B vaccine is given to one-tenth of the population each year. Even if we still have only 800 cases of ADEM each year, one-tenth of those cases would be expected to occur in people who happened to receive the hepatitis B vaccine sometime that year. Some of those 80 people would get ADEM soon after the vaccine, and they might assume the vaccine was to blame. Even if 90 or 100 vaccinated people developed ADEM in my hypothetical situation, it would be very difficult to know if it was just a bad year for ADEM by coincidence or if the vaccine was really causing some cases.

This is a complicated topic, and computer models are sometimes used to help elucidate the true risk of a vaccine when given to large populations. I think that you have very little to worry about. The risk of the hepatitis vaccine is very, very low. I recommend it to my patients all the time, and all my colleagues and I have been vaccinated. It is possible that a causal link will be established between this vaccine and a rare brain illness, but the risk will turn out to be very low.

Whenever we give any treatment, we have to weigh the risks and the benefits. If you have strep throat, there is a very slight risk that you will die from the penicillin or any other antibiotic. Yet we have to take the antibiotic to prevent the far more common complications of the strep throat.