The government's smallpox vaccination campaign is moving slowly: 1,043 health workers have been vaccinated out of a possible 500,000.

But duct tape is moving quickly: Homeowners building safe rooms have emptied stores despite protestations by officials that they are overreacting.

The great caution on one hand and high alarm on the other stem from the question no one seems able to answer: How much are we as a nation at risk?

As the country moves closer to war with Iraq and to the possibility of a biological or chemical terrorist attack, Americans' inability to determine the true degree of threat is leaving the nation confused, unnerved and distrustful of authorities.

"It is very frustrating to not be able to quantify the risk when there is so much at stake," acknowledged Dr. Julie Gerberding, director of the federal Centers for Disease Control and Prevention in Atlanta. "I cannot quantify the risk to my own satisfaction, and I am not going to qualify it by saying it is less than or more than. I know it is not zero. I believe there is sufficient risk to warrant this level of preparation."

The smallpox vaccination campaign, the first major anti-terror initiative to involve a large number of civilians, has become a focus for disquiet. It is dogged by questions that it was inspired, not just by anti-terrorism intelligence, but by partisan politics or the government's need to be seen taking action that will reassure Americans.

The campaign has foundered because many people are afraid of the vaccine's side effects and are concerned there is insufficient compensation for possible vaccine injuries. But there remains a deeper question among the people involved in the vaccination debate: Does the government know more about the level of threat than it is telling?

"There would be a benefit to being vaccinated if the threat of a smallpox attack were real," Dr. Ron Vorhoos of New Mexico said at a recent CDC training session. "But right now there is an uncertainty whether there is any benefit. There is only risk."

The risks from the vaccinations are substantial. The CDC estimates that for every 1 million vaccinations, there will be one to three deaths, along with 15 to 52 life-threatening reactions and up to 1,000 milder ones.

To balance fears of side effects, public health experts say the government should be supplying more detailed information on the true risk of a smallpox attack. They argue this could be done without imperiling intelligence sources.

"Since 1995 there has been nothing new, either that Saddam Hussein has [the smallpox virus] or that, if he had it, he would use it," said Dr. J. Michael Lane, former director of the CDC's smallpox eradication program. "If they had such information, I cannot imagine they would not immediately release it to the public. It would be immoral not to. And it would prove to the United Nations and to NATO that this is a dangerous man."

Other authorities counter that even partial information about a possible threat is enough to warrant action.

"If on Sept. 10, 2001, you had brought together the world's experts in terrorism and asked them the likelihood of someone using an airliner as a missile to take down a skyscraper, the conclusions would have been: It has never happened, airport security is in place, and we have no imminent risk information," said Dr. Michael Osterholm, a bioterrorism adviser to Health and Human Services Secretary Tommy Thompson. "Yet on Sept. 11, it happened. Today, we have enough risk information to know that a smallpox attack is clearly a possibility. If smallpox were to occur in this country, even a few cases, the fear and panic would be catastrophic. They would shut our economy down."

Despite the extremely slow pace of vaccination, Americans are becoming more unnerved by the prospect of smallpox. In a Gallup Tuesday Briefing Poll released this week, 63 percent said they were worried about possible smallpox attacks; 15 months earlier, 53 percent said they were concerned.

At the same time, Americans surveyed in the poll shared health care workers' skepticism about vaccination: 53 percent said they felt hospitals were doing the right thing when they refused to ask their workers to volunteer for the vaccination.

Some people think the vaccination plan was based as much on politics as science.

Over the 14 months that government officials debated whether to begin vaccination, the number of health and emergency workers deemed necessary by government officials grew from 15,000 to 500,000 to 10.5 million.

Much of the push to increase vaccinations came from Republican members of Congress, including new Senate Majority Leader Bill Frist of Tennessee, who is the only physician in the Senate, and from conservative organizations that argued the government could not deny Americans a vaccine their taxes paid for. Vice President Dick Cheney is said to have been the most influential voice arguing for higher vaccination numbers, which in one proposal opposed by the CDC would have opened the shots to the entire U.S. population.

One reality that may be driving the vaccination campaign is simply that it is one positive, practical action the government can take against a perceived terrorist threat.

Of all the most-feared biological agents, which range from tularemia to anthrax to Ebola virus to plague, smallpox is the only one for which there is a vaccine that can be administered easily and creates lasting immunity.

The smallpox vaccination campaign consists of what the government initially termed Phase One: vaccinating smallpox-response teams made up of state health department workers and emergency room and intensive care staff.

The initial phase won qualified support from major public health and medical organizations, which acknowledge the need while reinforcing concerns about compensation and side effects.

Phase Two, which could vaccinate 9.5 million more Americans, has not won similar endorsements.

"Because the risk is not zero, because there is evidence that people we do not trust may have smallpox, and because we now have experience with bioterrorism in this country, it makes sense for us to be better prepared than we are today," said Dr. Georges Benjamin, executive director of the American Public Health Association.

"But as we begin to get into more and more workers, the risk of more people getting sick, the resources it takes, the funds diverted from diseases that are problems today, for me makes it a fuzzier discussion. Before we get to a larger group, we will need to reassess all the assumptions."

At that point, which by the government's original timetable could come within a month, calls for more precise information about the threat of bioterrorism will become louder.

"Do we have it exactly right? Should we be more prepared, or are we overdoing it? We won't know until it either happens or it doesn't," Gerberding said. "But given everything we know right now, I think it would be irresponsible not to have an enhanced level of protection."