A new study shows that the United States may be less vulnerable to a possible bio-terrorist attack using the smallpox virus than previously thought.

The study conducted by scientists at Oregon Health and Science University shows that the vast majority of Americans vaccinated against smallpox more than 25 years ago may still have a substantial level of immunity to the fatal virus. Some 120 million Americans were vaccinated against smallpox more than 30 years ago. Until now, the vaccine has been assumed to offer its best protection for only three to five years.

The university study also concluded that in the long run, repeated vaccinations do not result in a higher level of disease protection. The researchers studied more than 300 people who had been vaccinated, some several times.

But some experts say they are not reassured by the new study. Anthony Fauci, Director of the National Institute of Allergy and Infectious Disease cautioned that the relationship between immune response and real protection from disease is still not well understood. He also noted that the finding does not address the fact that half of American have never been vaccinated for smallpox.

Currently, the U.S. government requires about one half million military personnel to get the smallpox vaccination.

The Bush administration is also seeking to voluntarily immunize several million medical and emergency personnel who could be immediately exposed to smallpox in any outbreak. However, fewer than 40,000 workers have so far volunteered to receive the vaccination.

Smallpox was declared eradicated in 1979, but the U.S. government believes some groups may have developed the virus for use as a biological weapon. The United States and Russia kept samples of the virus.

A panel of U.S. scientists recently suggested that most Americans should not get vaccinated for smallpox unless they remain under medical supervision after the inoculation.

The panel emphasized that the risk of a biological attack using smallpox is only theoretical, while noting the side effects carry greater risks than other inoculations.