The Bush administration is considering a mass vaccination campaign against smallpox, amid concerns that the deadly virus could be used as a terrorist weapon. But researchers report this week that so-called targeted vaccination would work almost as well as a mass vaccination program.

Concern about a possible bioterrorist attack using smallpox grew after the September 2001 terrorist attacks in the United States and the subsequent mailing of deadly anthrax spores.

The World Health Organization declared smallpox eradicated in 1980. But strains of the lethal virus still exist in guarded laboratories in the United States and Russia. And, there is concern that other countries could have undisclosed stocks of the virus, and that terrorists may try to get hold of it.

There is no cure for smallpox, which kills up to a third of its victims.

The Bush administration is considering a plan to vaccinate everyone in the United States. But health officials say the vaccine is not without risk, and can cause life-threatening complications and even death in some cases.

The administration is also weighing a proposal of targeted vaccination, that is, to vaccinate only those people who come in close contact with infected individuals.

Ira Longini, a professor of biostatistics at Emory University in Atlanta, Georgia, prefers targeted vaccination to mass vaccination.

"Mass vaccination would be effective, but you could have a fairly large number of deaths and complications due to vaccine. And targeted vaccination could be almost as effective," he said. "You would have far fewer complications to the vaccine, because you're vaccinating far fewer people."

In an article in the journal Science, Mr. Longini and colleagues write, their conclusion is based on a model of 2,000 people in a variety of settings, including work, school and daycare centers.

Professor Longini says smallpox is spread through close, as opposed to casual, contact with infected individuals. And, he says, there is no mistaking the illness, which is characterized by bumps, or pox, on the skin. Also, Professor Longini believes, targeted vaccination would be effective because of how slowly smallpox spreads.

"You get generations of cases roughly every two weeks," he said. "So, it's not like influenza, for example, which spreads rapidly. It develops relatively slowly."

The model also assumes 20 percent of uninfected people exposed to smallpox would not come forward immediately for vaccination. Professor Longini says that is not necessarily a recipe for disaster.

"I think the uniqueness of smallpox, that it's easily identifiable and that it spreads relatively slowly and just mostly to close contacts, makes the strategy effective," he said.

Jim Koopman, a professor of epidemiology at the University of Michigan at Ann Arbor, reviewed the study and believes targeted vaccination is preferable to mass vaccination, which he says is unlikely to thwart a terrorist assault using smallpox.

"In mass vaccination campaigns, people come voluntarily to the clinic," he said. "But that always leaves pockets of unvaccinated individuals. And these pockets can sustain chains of smallpox transmission. So, mass vaccination is not a magic bullet that can stop transmission. The key to stopping smallpox is to identify contacts that could carry transmission. And those people must be vaccinated or quarantined."

President Bush is expected to make a decision soon on a smallpox vaccination plan to protect Americans, should an attack ever occur.