he federal government's latest plan for dealing with a smallpox attack underscores how difficult that task would be. Officials suggested this week that even a single case of smallpox could trigger a nationwide program of voluntary vaccinations. The first priority would be to isolate and care for the sick and to vaccinate anyone who had close contact with them. But the federal Centers for Disease Control and Prevention has also instructed state and local governments to prepare for emergency, large-scale vaccinations if needed.

The guidelines are breathtakingly ambitious. They outline steps by which the states would establish clinics to rapidly screen, evaluate, counsel and vaccinate an average of 100,000 people a day, or a million in 10 days, by operating 16 hours a day. That task would require 4,680 health workers and volunteers. A much broader effort to vaccinate virtually the entire population after an attack could require more than a million health workers and volunteers. The speed and scope of the undertaking has public health experts gasping.

The daunting task reinforces our belief that more thought should be given to vaccinating large numbers of people now, on a voluntary basis. Vaccination would be far more orderly and systematic if carried out in advance rather than in the chaos of a bioterror attack, and those at risk of serious side effects could be screened out more carefully. The creation of a large pool of vaccinated Americans would lessen any damage from a later smallpox attack and thus make it less likely that any terrorist would even resort to smallpox as a weapon.

The main danger is that a portion of the American public is vulnerable to severe side effects — including death, permanent disability, severe skin eruptions and scarring — either from taking the vaccine or from coming into contact with those recently vaccinated. The degree of danger is difficult to gauge, but the number of people vulnerable to harm is probably greater today than it was when smallpox vaccinations were last routinely administered. There are a lot more people today whose immune systems are compromised or who have suffered from skin disorders that might put them at risk. One scientific article suggested this month that up to half of the American public should be discouraged from getting smallpox vaccine.

But there are ways to mitigate these problems. Medications to combat many of the side effects will need to be produced in quantity, and special bandages may be able to prevent spreading the virus from the vaccination site on the arm. A weaker but safer vaccine may be made for those at risk.

The federal government is expected to announce soon its plans for vaccinating health and emergency personnel who would have to cope with any smallpox outbreak. The administration would be wise to make this group as large as possible and to monitor it closely to determine the real risk of side effects and how best to prevent harming those in close contact with the vaccines.