US biodefence experts are divided this week over a research paper that calls into question a government decision to spend hundreds of millions of dollars on smallpox vaccine.

The paper's authors say that 40 million doses of vaccine is enough to protect the country from a terrorist attack. Published in the current issue of Emerging Infectious Diseases (7, 959–969; 2001), the paper was written by Martin Meltzer, an economist at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, with J. Donald Millar, who led the CDC's effort to eradicate smallpox, and two CDC biologists.

Health secretary Tommy Thompson announced on 28 November that the United States will buy 155 million doses of vaccine from Acambis, a British firm, for $428 million, bringing the total effective US stockpile to 286 million doses.

Government scientists defended the purchase. Donald Henderson, who led the global effort to eradicate smallpox and now advises Thompson on biodefence, suggests the authors were influenced by the CDC's earlier decision to hold 40 million doses.

“Amazingly, they reached the conclusion that this indeed was the correct number to purchase,” Henderson says.

Other researchers dispute the paper's assumption that every patient would infect an average of three others. Steve Leach and Raymond Gani, of Britain's biological weapons laboratory at Porton Down near Salisbury, report in Nature this week (pages 748–751) that each smallpox patient is likely to infect from 3.5 to 6 others, and even more initially, because people now lack immunity to the disease, which was eradicated worldwide in 1976.

But Meltzer says his team's work was rigorous. “As researchers, my colleagues and I try to approach the analysis of each and every problem in a neutral fashion, employing scientific procedures,” he says.

There is concern that stockpiling vaccine is a waste of scarce public-health resources, as a move to vaccinate the entire US population would result in hundreds or thousands of deaths from side effects. But some officials who agree the stockpile may not be needed argue that it still reassures the public. “It may be politically necessary and prudent to make sure that we allow a large safety margin,” says Stephen Morse, director of the Center for Public Health Preparedness at Columbia University in New York.