Diluted smallpox vaccine is potent. (Biomedicine).

For anyone concerned that terrorists might use the dreaded smallpox virus as a weapon, there was a double dose of good news last month.

First, two studies led by investigators at the National Institute of Allergy and Infectious Diseases in Bethesda, Md., found that the 15.4 million doses of smallpox vaccine held by the U.S. government can be diluted to one-tenth their original concentration and still be effective for immunizing people. Conducted on more than 700 healthy volunteers, the dilution studies will appear in the April 25 New England Journal of Medicine (NEJM). They were launched after the Sept. 11 terrorist attacks on New York and Washington because of concern that the U.S. stockpile of vaccine was inadequate for thwarting a smallpox attack.

The second piece of welcome news came from the French firm Aventis Pasteur, which confirmed that it still has about 75 million doses of a smallpox vaccine created decades ago. Initial tests show that the doses remain potent, and Aventis Pasteur has offered the stockpile to the U.S. government if it will limit the company's liability for any side effects in people who receive the vaccine. The United States has also contracted with Acambis of Cambridge, England, to produce more than 200 million new doses of the smallpox vaccine.

The rapidly expanding vaccine stockpile has raised a difficult question, however. The current policy of the Centers for Disease Control and Prevention is to use the vaccine only during an actual smallpox outbreak, and then just on people in the vicinity of the outbreak. However, some scientists and public health officials now argue that there should be a preemptive mass vaccination of the entire country. The drawback is that the vaccine can have serious flulike side effects and kill about one in a million people getting it.--J.T.