Health officials say that by year end there will be enough smallpox vaccine to immunize all Americans, but experts disagree about what to do with it. The question centers on whether the theoretical risk of a smallpox bioterror attack in the US justifies the known risk of vaccination side effects, which in rare cases can be fatal. Smallpox itself kills about one third of patients and leaves many survivors scarred.

According to analysts at the Cato Institute, a public policy think-tank in Washington, DC, the answer is a voluntary vaccination program, in which Americans are fully informed of the risks.

Even if only part of the population opts for vaccination, that would help protect the whole population from the contagious infection--and would render smallpox a "less attractive" biological weapon, Charles V. Pena, a co-author of the Cato Institute's report, noted in an interview with Reuters Health.

Not everyone agrees that a pre-emptive vaccination program is the way to go, largely because the risks of vaccination could outweigh the benefits. Before smallpox was eradicated and mass vaccination ended in the US in 1972, there were several deaths per year associated with the vaccine. Smallpox vaccine contains vaccinia that, in a small number of cases, can trigger serious skin or systemic reactions.

The current smallpox response plan of the US Centers for Disease Control and Prevention (CDC) employs a "ring-vaccination" strategy, in which patients with suspected or confirmed smallpox will be isolated and their contacts traced, vaccinated and watched closely. The vaccine can be given shortly after exposure to the virus to treat infected individuals and contain outbreaks.

But Pena argued that this strategy, used in the past to contain natural smallpox outbreaks, is not suited for a bioterror attack on a large, mobile population.

"We're not talking about a naturally occurring outbreak," he said, noting that a coordinated terrorist attack could take place at multiple sites, and not be apparent until many people have been exposed. Before the characteristic rash and pus-filled lesions of smallpox appear, the infection's initial symptoms resemble those of the flu.

Of course, a bioterror attack using smallpox remains a "theoretical threat," Pena noted. The only known existing samples of the virus are at the CDC and in a repository in Russia. However, officials worry that other governments or terrorist groups could have samples as well.

"I don't see that there's any reason to deny the vaccine" to the public, Pena said.

The CDC ring-vaccination plan is currently under review. In an interview last month, Dr. Anthony Fauci, head of the US National Institute of Allergy and Infectious Diseases and a key presidential advisor on bioterror, said that it is time for a public debate about mass smallpox vaccination.

"The current ring-vaccination approach may actually be the best strategy," Dr. Fauci said. "However, there are enough concerns and questions about the current plan...that I think it would be helpful and productive to have an open debate about the best policy."