The bad news comes from a study of 621 microbiologists in Maryland who received fresh vaccinations between 1994 and 2001 to protect them in their work. Only about 40, or just 6 per cent, were still immune from their earlier vaccinations.
"The study is, to the best of my knowledge, the only one since eradication which tries to look at the durability of immunity," says lead author Michael Sauri, director of the Occupational Medicine Clinic in Maryland. "It's showing us that after 20 years immunity is not going to be there."
In the US, for example, about 60 per cent of the population has had a smallpox vaccination. The study suggests that most of these people are now just as susceptible to smallpox as the 120 million born since the government halted vaccination in 1972.
That strengthens the case for pre-emptive immunisation, some experts think. "It adds to the argument that you can't count on any protection we thought we had," says Bill Bicknell of Boston University, a former commissioner of the Massachusetts Department of Public Health who has been arguing for mass vaccination since the anthrax attacks, in case terrorists try smallpox next.
He thinks that outbreaks would be much easier to contain if almost everyone is vaccinated. "I'm not saying you just go straight in and vaccinate the population-you'd do it steadily in stages," Bicknell says. Healthcare workers would be first, followed by volunteers screened to check they're healthy.
But extrapolating from figures from the last mass vaccination in 1968 suggests at least 180 people would die of complications. That's a high price to pay for protection against an attack that may never happen.