Officials with the U.S. Centers for Disease Control and Prevention say it's not necessary, yet 50 percent of citizens polled by ABCNEWS say they would still get the shot if and when the vaccine becomes available. We asked vaccine expert Dr. Steven Black to explain why nationwide smallpox vaccinations are not recommended.

Immunizations are among the most widely used and effective public health measures. Many immunizations in current use, including those for hepatitis B, polio and whooping cough, have been developed to replace earlier vaccines and provide a more acceptable safety profile.

Because of the continuous safety review process and the application of new technologies in vaccine development, the vaccines we currently use routinely are more effective against more diseases, and are safer than ever.

However, vaccines are not always without dangers.

The vaccine for smallpox was developed at the end of the 18th century and was last routinely used 30 years ago, in the 1970s, before the disease was eradicated worldwide in 1977. The vaccine provides protection against the dreaded risk of smallpox — a disease that killed one out of three people it infected and left most others with lifelong scars or disabilities.

Because of the high risk of smallpox disease and the limits of vaccine technology in the first half of the 20th century, people accepted the dangers associated with routine smallpox vaccination, which were more than outweighed by the ever-present threat of smallpox death.

Small, But Significant Risk of Death

Unfortunately, the smallpox vaccine is just not as safe as any of the other vaccines routinely used in the United States today.

The vaccine injection causes a red, tender and crusting reaction at the vaccination skin site that lasts up to two weeks.

More importantly, one out of 150,000 smallpox vaccination recipients experiences more severe reactions, including overwhelming infection due to the vaccine virus in individuals with abnormal immune systems, encephalitis or brain infection. Another one out of 500,000 individuals will die as a direct cause of the vaccine.

Although the risk of either death or these severe side effects may sound relatively rare, vaccination of the entire U.S. population would result in 600 deaths and 2,000 individuals with serious brain infections. These very real risks must be balanced against what is currently only a theoretical risk of smallpox being introduced by terrorists.

U.S. Smallpox Plan Is Effective

Apart from the hazardous side effects, another reason not to recommend nationwide prophylactic vaccinations is the strong likelihood that the disease can be restrained and managed if an initial case is identified.

The strategy U.S. health officials plan to use is to vaccinate individuals in a "ring" around any cases that are identified, including family, friends, and co-workers. This strategy will effectively control and eventually eliminate infection while exposing the smallest number of people possible to the risks of vaccination.

In addition, it is known that individuals exposed to smallpox can be protected against illness if they are vaccinated within a few days after exposure. Therefore, we have no need to expose the entire U.S. population to the risks of smallpox vaccination with the current vaccine.

It makes much more sense to stockpile enough vaccine to vaccinate only when and if the threat becomes real. This vaccine stockpile can serve as an effective deterrent against terrorism and buy us the time that is needed to develop a safer smallpox vaccine that could be acceptable for general use.

Dr. Steven Black is co-director of the Kaiser Permanente Vaccine Study Center in Oakland, Calif.