Despite President Bush's pledge to make the smallpox vaccine available to anyone who insists on it, it remains extremely difficult for the average American to be inoculated against the deadly disease. Federal health officials are focused on Bush's chief goal of immunizing millions of medical personnel and emergency responders who would serve as a sort of domestic front line in the event of a biological attack.

On the spectrum of threats confronting the nation today, officials and security analysts believe that a smallpox attack is a low-probability, high-impact risk. Only two countries -- the United States and the former Soviet Union -- are known to possess the variola virus, but intelligence experts suspect that a handful of hostile nations or terrorist groups may have acquired stocks, which they could spread in aerosolized form or through person-to-person contact.

For that reason, 50 years after the last case of smallpox was seen in the United States, Bush announced on Dec. 13 an unprecedented campaign to resume vaccinations. The first step was the mandatory inoculation of 500,000 military personnel, including Bush as commander in chief.

On the civilian side, the administration aims to improve response capabilities while minimizing risks of the vaccine, according to Julie Gerberding, the physician who runs the Centers for Disease Control and Prevention in Atlanta.

Officials had hoped by March 1 to have inoculated about 500,000 medical volunteers who would treat initial cases and open mass vaccination clinics during an outbreak. But as of Feb. 28, only 12,690 volunteers had come forward. A second phase calls for up to 10 million police, fire and other emergency workers to be inoculated.

This effort has gotten off to a slow start largely because of concerns about the risks of the vaccine and the lack of compensation for anyone injured by it. Side effects range from the unpleasant (malaise, fever, rash) to the extreme (blindness) to the fatal (encephalitis). For every 1 million people vaccinated in the past, 14 to 52 suffered the most serious complications, with one or two deaths.

An additional 1,000 experienced milder side effects. That's about one-tenth of 1 percent.

Compared to the potential toll of the disease itself, those numbers are small. With no known treatment, smallpox kills one in three people infected and leaves many others permanently scarred. Its toll has been breathtaking: 500 million deaths in the 20th century.

Before 1971, smallpox vaccines were routine -- immunization was required for schoolchildren, and adults traveling overseas often received a second, or booster, inoculation. Today, about one-third of Americans have never been immunized, and it is unclear how much protection remains for people who were inoculated decades ago.

The vaccine, which is made from a live virus related to smallpox, is considered especially dangerous for people with weakened immune systems, such as those infected with the AIDS virus or recipients of organ transplants or cancer therapy such as chemotherapy. Pregnant women, nursing mothers, infants and people with severe skin problems should not receive the vaccine -- and neither should anyone who has close contact with people in those categories, because the virus in the vaccine can spread accidentally.

Few practicing physicians have treated smallpox, leading officials to worry that the greatest challenge may be knowing when an attack has occurred.

Doctors who led the campaign that eradicated the disease globally found that the vaccine provided excellent protection even when administered three or four days after exposure. But for the first 7 to 17 days after exposure, patients are unlikely to exhibit symptoms and are not contagious. The first signs are generally fever, aches or lethargy. Next comes a rash that initially appears similar to chickenpox but soon worsens. Patients may pass on the disease through contact until all scabs have fallen off, or about one week after the rash appears.

If an outbreak occurs, the CDC plans to isolate and vaccinate family members, co-workers and others who come in close contact with the first cases. Health officials would have the authority to quarantine if they feared that people exposed to the virus had not come forward for vaccination.

If a single case of smallpox is confirmed, officials say, they will assume that it is the result of a criminal act and move quickly to immunize all 280 million Americans.

Sufficient supplies of vaccine are on hand, and the government is purchasing newer stocks and underwriting research on safer, second-generation vaccines.

Each state has submitted plans for inoculating its entire population within 10 days, but most analysts predict the system would be overwhelmed.

Although the administration recommends against vaccination for the general public, Bush has ordered staff to make it available to any American who insists on it. David Fleming, deputy director of health science at CDC, recently said the federal government would take on that chore, perhaps by contracting with academic medical centers. It is likely that anyone demanding the vaccine will be required to sign waivers releasing the federal government from liability for adverse events.

However, the plans for offering vaccination to the public remain sketchy, and the current Bush budget does not set aside money for the program. The only other available vaccination route is through one of the many clinical trials being run by the National Institutes of Health and vaccine makers.

John Medici, a Prince William County hazardous materials officer, gets his smallpox vaccination. Millions of emergency workers are due to join him.