outine smallpox vaccinations were discontinued in the United States in 1972, and the disease was declared eradicated in 1980. But now, faced with the possibility of a terrorist attack, the federal Centers for Disease Control and Prevention have adopted guidelines for a mass vaccination in the event of an outbreak.

The vaccine carries some risk of serious complications. But the C.D.C. says anyone who has definitely been exposed to the virus should be vaccinated. For those not yet exposed, or uncertain whether they were exposed, the picture would be more complicated. Following are questions and answers about the government’s plan.


THE RISKS

What are the complications? In the 1960’s the rates of severe side effects were about 12 cases of encephalitis and 39 cases of severe eczema per million vaccinations. The death rate from complications was about 1 per million. Milder but more common side effects include fever, sore arms and swollen lymph nodes.

What if I’ve already been vaccinated? It is not known how long a childhood vaccination will protect an adult, so the C.D.C. recommends vaccination for anyone who has definitely been exposed to the virus, regardless of age or medical condition.

Should anyone avoid vaccination? Health officials say some people should avoid the vaccine unless it is certain they have been exposed to smallpox.
Children younger than 1 Infants have a greater risk of complications from the vaccine. About 42 children out of a million will experience brain swelling, which can lead to retardation or death. Small children are also more likely to touch their vaccination sore and then their eyes or mouth, causing new sores.
Pregnant women The risk from smallpox vaccine to the fetus is relatively small, but health officials recommend that pregnant women who have not been exposed to smallpox wait until after childbirth. If they have been exposed, vaccination is recommended.
People with weakened immune systems When smallpox was declared eradicated in 1980, little was known about immune-system problems, but experience with other serious infections suggests that such people will be more vulnerable, both to smallpox itself and to severe side effects from the vaccine.
The C.D.C. says those who have not been exposed to smallpox ‘‘should think about not being vaccinated or waiting to be vaccinated until you have completed any treatments that affect your immune system function.” The waiting period can be as long as three months.
People with skin conditions Vaccination creates a high risk for a severe skin condition called eczema vaccinatum, which has been compared to third-degree burns all over the body. People with a history of eczema should not be vaccinated unless they know they have been exposed to smallpox. Those with allergic reactions, severe burns, impetigo and chicken pox should wait until the condition clears up.

What precautions follow vaccination? The vaccinia virus in the vaccine is a cousin of the smallpox virus. Studies suggest that those who have been vaccinated rarely transmit it to anyone else. But they should not let anyone — especially children — touch the vaccination spot until it has healed and the scab has fallen off.

How are complications treated? Two treatments exist for severe side effects: vaccinia immune globulin, which is derived from the blood of donors vaccinated with smallpox vaccine, and cidofovir, an antiviral drug marketed as Vistide. At the moment, there is only enough globulin for about 600 patients, the number of serious complications expected from vaccinating five million people. More is being produced.

THE GOVERNMENT’S PLAN

What would happen first? The C.D.C. is playing down the possibility of a massive rush to vaccinate and anticipates providing vaccine to state and local health officials to carry out their own plans. But if a single case is confirmed “we will act as if the nation were under attack” unless it clearly stems from a lab accident, a C.D.C. doctor said. In an outbreak, the president could declare a health emergency and take control of the vaccination effort.

Would vaccination be mandatory? Under the new plan, vaccination is voluntary even if someone has definitely been exposed to the smallpox virus. But anyone who has been exposed and refuses vaccination may be involuntarily quarantined for up to 18 days.

Must I consent to be vaccinated? Because all smallpox vaccines are considered “investigational new drugs,” written consent is required. The C.D.C. envisions showing all patients a video about the disease, vaccination and side effects. Medical personnel will then ask for a medical history to screen out those who should not be immunized.

Is there enough vaccine? About 155 million vaccine doses are available now, and enough for all 280 million Americans should be ready by year’s end. The vaccine is not available through private doctors, only through the C.D.C. The procedures for using such drugs are complex, but would be streamlined in an emergency.

What are the logistics? Mass vaccination would require enormous resources. Clinics would take over schools, warehouses, stadiums. Huge numbers of workers would be needed to give injections, screen patients, get consent forms and control crowds. Isolated hospital wards would have to be set aside for smallpox victims and those suffering complications, and to quarantine those who refuse vaccinations.

What about the cost? The C.D.C. estimates $5 to $10 a patient, but that covers only the screening and injection itself.