he White House's smallpox vaccination plan is a sensible approach to protecting the nation against a possible bioterrorist attack. It will start with the two groups that most need protection — military troops and civilian health personnel who would respond to any smallpox attack. Then it will make the vaccine available to the general public without recommending that anyone take it. With mounting concerns over the safety of the vaccine, and with enormous uncertainty over whether rogue nations like Iraq or terrorist groups like Al Qaeda can launch a smallpox strike, this step-by-step approach has much to recommend it.

The vaccine will be given first to 500,000 military service members, including health units and troops that might be involved in military action against Iraq. The smallpox virus would have little value as a tactical battlefield weapon — it takes 10 to 14 days before making people sick — but it could sicken and terrorize unvaccinated troops who remain in the area for some time.

The vaccine will also be offered on a voluntary basis to some 435,000 or more civilian health care workers and emergency responders who would be the first to encounter contagious smallpox victims after an attack. This is a prudent step because hospitals have historically magnified the spread of smallpox, with one sick patient infecting others as well as the medical staff. Health and emergency workers need vaccination for their own protection and to lessen the likelihood that some might flee in terror from smallpox patients, just as some doctors once refused to care for AIDS patients.

In a second phase, the vaccine will be offered to some 10 million additional health and emergency workers. If enough accept, they could provide a sizable group in which to assess adverse reactions.

President Bush went out of his way yesterday to discourage the public from taking the vaccine. He stressed that there was no evidence that a smallpox attack is imminent, and said the government was not now recommending mass vaccination. Citizens who "insist on being vaccinated" will be accommodated, possibly starting in late spring or summer next year.

The main protection for the public right now is a stockpile of vaccine that could be administered to everyone after any attack. There is enough to inoculate everyone, the president said, and the vaccine can protect people who receive it up to four days after exposure to the virus. But there is some doubt as to how quickly the vaccine could be distributed after an attack. Mr. Bush is right to give well-informed citizens access to the vaccine in advance if they want it.