PRESIDENT BUSH has announced plans to allow voluntary vaccination of all who want it, but some things are worth remembering in deciding whether to volunteer:

Smallpox is a very nasty disease. The variola major form of the virus, which affects 90 percent of those who have contracted the disease, kills one-third of its victims.

There is no proven treatment once you are ill with smallpox.

Smallpox generally spreads slowly. In a crowded municipality in Bangladesh in which I worked in 1972, only 3 percent of inhabitants became infected, the vast majority before we began vaccine control activities.

The risk of our being exposed to smallpox is unknown (and unknowable).

The virus no longer exists in the wild. Only two countries are known with certainty to have stores of the virus: the United States and Russia. We have not been given evidence that other countries or individuals possess the virus and, if they do, whether they would attack us.

There are several points worth remembering about the smallpox vaccine:

It is very nasty. Healthy individuals will develop sore arms and weeping blisters and will shed virus capable of infecting others. Anyone with a depressed immune response (from HIV, chemotherapy, organ transplantation, etc.) or a predilection for skin disorders such as eczema stands a significant risk of developing the life-threatening disease, including those who were inadvertently infected by someone who was recently vaccinated.

The vaccine is highly effective in preventing disease and death from smallpox, even when administered four to five days after a person becomes infected. I know, because I conducted one of the seminal investigations that established this fact.

It makes sense to vaccinate a small core group of public health, hospital and other first responders. They will feel safer responding to potential cases. They can be carefully screened and followed to minimize complications and secondary spread of the vaccine virus. Perhaps most importantly, that provides training and tests our plans and procedures for mass vaccination, should that ever become necessary.

Given the unknown risk of ever being exposed to smallpox, and the known risks of complications from vaccination (particularly the potential for inadvertent spread to those at high risk of life-threatening disease), one can justify nearly any position regarding voluntary mass vaccination.

But it would seem to make little sense to risk vaccine complications in the absence of a known threat so long as effective plans and procedures are in place for responding to an introduction of smallpox.

Alfred Sommer, a medical doctor, is dean of the Johns Hopkins Bloomberg School of Public Health.