mericans feeling vulnerable to a potential smallpox attack by terrorists can breathe a little easier, thanks to two encouraging developments last week. New experimental results showed that the federal government's small stockpile of smallpox vaccine could be enlarged fivefold through dilution without losing much effectiveness, providing enough medicine to protect some 77 million Americans. At the same time, officials revealed that a drug company had a large additional supply of vaccine — some 85 million doses — left over from decades ago that might also be expandable through dilution. The combined result may mean that the government could quickly marshal enough smallpox vaccine now to protect most of the country should the need arise.

On top of that, the federal government has contracted with drug companies to deliver by the end of this year 209 million doses of a second-generation vaccine made by more modern techniques. Clearly, the days when the nation's 280 million residents were virtually defenseless against a scourge that spreads easily and kills some 30 percent of its victims are over. The only question now is how best to use the vaccine.

Government officials consider the risk of a smallpox attack quite low because the only known stocks of the virus are held at guarded sites in the United States and Russia. But there are persistent rumors of clandestine stocks that terrorists might be able to tap. So officials consider it prudent to be ready for the worst.

The current plan, fashioned by the Centers for Disease Control and Prevention, calls for holding the vaccine in reserve until after an attack occurs and then rushing it to the scene. Health authorities would identify and isolate infected individuals and quickly vaccinate all those who had come in contact with them, a process that could prevent the spread of disease.

That approach has worked well in containing natural outbreaks in the past. The main drawback is that the virus might move too fast to keep it contained, particularly if attacks were launched simultaneously on multiple cities.

The current generation of smallpox vaccines is undeniably riskier than other vaccines commonly taken by the public. It can be expected to kill one or two people among every million vaccinated and cause serious side effects in hundreds of others. If the entire population was vaccinated, there could be a few hundred deaths and tens of thousands of adverse reactions. That makes it extremely difficult for health authorities to recommend mass vaccination absent compelling proof that an attack is likely.

But there seems to be no reason why the vaccine should not be available to those who want it. People who fear the side effects and think that the likelihood of an attack is remote could shun the vaccine. Those willing to accept the risks should be able to get the vaccination, as Americans did routinely back in the days when smallpox was still rampant in the world.

If enough people took that option, the likelihood that terrorists would resort to smallpox would diminish. This is a case in which the government should provide the best possible estimate of the risk of attack and the risk of vaccination and let informed members of the public choose their own medicine.