resident Bush faces a momentous public health decision: whether to make smallpox vaccine available to all who want it or limit its use to the health personnel who would have to cope with any bioterrorist attack. The issue is fraught with difficulties. If Mr. Bush makes the vaccine widely available, there will be severe side effects in a small percentage of recipients. If 100 million people chose to be vaccinated, perhaps 100 of them might die and many more could be severely injured. But these casualties would pale in comparison with the toll that would be exacted by a smallpox attack. Should a smallpox epidemic spread out of control, it could kill tens of thousands or even hundreds of thousands of unvaccinated people.

Decades ago, when smallpox was still rampant, the decision to vaccinate was easy. Smallpox was such a great risk, killing some 30 percent of those it infected, that the vaccine complications seemed acceptable. In 1968, some 14 million Americans were vaccinated, and only 9 died from the side effects. But as smallpox was being eradicated around the globe, routine vaccination was dropped here in 1972.

Once smallpox was eradicated, all supplies of the virus were supposed to be concentrated in two well-guarded laboratories, one in the United States and one in Russia. No one, however, can be sure that all the virus was turned in, and no one is certain whether impoverished Russian scientists may have sold some to rogue nations.

Whether Saddam Hussein has a clandestine stock of smallpox, along with his known supplies of other germ weapons, is uncertain. After the Persian Gulf war, United Nations inspectors found a freeze dryer labeled "smallpox" in Arabic. Iraq claimed that it was used to store vaccine, but a top Iraqi scientist admitted working with camelpox, a related virus. A smallpox outbreak occurred in Iraq in the early 1970's, offering Iraqis a chance to sequester the virus for weapons purposes even without getting illicit stocks from Russia.

Vaccination would be an easy call were it not for the problem of side effects. Top federal health officials estimate that the current generation of vaccines would cause life-threatening side effects in 15 of every million people vaccinated, and 1 or 2 of them would die. That implies that if, say, 100 million Americans chose to be vaccinated, some 1,500 would develop life-threatening complications and 100 to 200 might die. Many others could become blind or permanently scarred.

The reaction rates might be higher today than in the past because more people today — perhaps 30 million to 50 million — would be vulnerable to the side effects. That includes people whose immune systems have been compromised by various diseases and drug treatments as well as people suffering from certain common skin conditions. These individuals could always choose not to be vaccinated. But even that might not protect them completely from secondary infection at the hands of those who get the vaccine, which contains a live virus called vaccinia, a smallpox relative.

Anyone who touches the vaccinated area on someone's arm or a contaminated bandage can spread the vaccinia virus. People who scratch the area and then touch their eyes can sometimes make themselves blind. All these risks will need to be managed and minimized.

Any large-scale vaccination campaign will require health professionals to communicate the risks clearly. It will also impose a duty on those who are vaccinated to wash their hands regularly, wear bandages and long-sleeved shirts to cover their vaccination sites, and take care to avoid infecting innocent bystanders. Researchers hope to develop bandages that are especially good at containing the virus.

The Pentagon plans to vaccinate up to half a million troops shortly. For civilians, federal health officials favor a phased vaccination program that would offer the vaccine first to some 500,000 health care workers, then to as many as 10 million health care and emergency workers, and finally, perhaps in 2004, to the entire population. That strikes us as reasonable. As long as informed individuals can decide for themselves whether to take the risk, and as long as precautions are in place to minimize secondary transmission, mass vaccination seems a prudent way to protect against a smallpox attack, and possibly even deter it.