The U.S. Centers for Disease Control and Prevention have failed to properly reassure the American public of the safety of the smallpox vaccination plan, leading to a growing backlash by health care providers who want nothing to do with it, an expert at Boston University charges.

``The CDC has not fairly laid out how it can be done safely and what the real historical risks are,'' said William Bicknell, a smallpox expert at the BU School of Public Health.

Since the plan was announced, a growing number of hospitals, doctors and nurses have said they will not participate, citing safety risks.

In Massachusetts, Cooley Dickinson Hospital in Northampton has opted out of the plan, and several more are on the fence. The Massachusetts Nurses Association also has said it will urge its members not to be vaccinated.

Bicknell, who testified on the subject before Congress this week, blamed the CDC for the growing opposition to the plan. He said the agency let potential volunteers develop exaggerated fears, failed to assure them they would be protected from liability in case of bad reactions and did not publicize the positive experiences of the Israeli and U.S. military vaccination programs, which have had few problems.

Bicknell said that to ensure the success of the program, the Bush administration at the highest levels ``must make it clear to providers that this is very important to do.''

In addition, Congress must address the liability issue and the CDC must ``far more accurately and realistically represent the risk, which it has not done. They overplay the risk dramatically, and that scares the crap out of people.''

Bicknell said healthy adults have nothing to fear, and the chances of a health care worker passing infection on to a sick patient are also slim.

Although Bicknell backs the Bush plan, he has argued the case for mass inoculation before an outbreak, rather than waiting for an outbreak to occur. Such a plan would involve vaccinating millions of Americans.