Prior to 1972, all Americans were vaccinated against smallpox. The eradication of the disease was declared in 1980 and over the next several years, all countries stopped their national vaccination campaigns. Now, faced with the possibility of a terrorist attack, the federal Centers for Disease Control and Prevention (CDC) have adopted guidelines for a mass vaccination in the event of an outbreak.

What would happen first if an outbreak were confirmed?

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, the nation will act as if the nation were under attack unless it clearly stems from a lab accident. In an outbreak, the president could declare a health emergency and take control of the vaccination effort.

How does the disease progress?

First of all, if the vaccine is given within the first four days after exposure to the virus, it protects against smallpox. The virus progresses as follows:

• Week 1: Virus is introduced into the respiratory system, and replicates in the lymphatic system.

• Week 2: Halfway through week two, the first symptoms start to appear. Fever, backache, headache, nausea and malaise are the recognized symptoms. At the end of week two, the person infected becomes contagious for 17 days.

• Week 3: While contagious, a rash will appear. This rash will continue through week four and an additional four days, the recognized end of the illness.

• Week 4: Death can come from bleeding or tissue damage; about one in three with the full-blown infection will die.

• Week 5 and beyond: Survivors of the infection will bear scars from the rash developed in week three.

Who should avoid vaccination?

• 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 site and spread the virus, causing new sores and other complications.

• Pregnant women: The risk from smallpox vaccine to the fetus is 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 immune deficiency diseases: 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.

• People with eczema: These people are more susceptible to serious complications from vaccinia, the virus related to the smallpox virus that is used in vaccines.

What happens to people with eczema?

In people with eczema, exposure to vaccinia through the vaccine or by contact with someone who was recently vaccinated can cause a condition that can lead to scarring, blindness and even death. People with skin problems are at risk for a condition called eczema vaccinatum, which can cause high fever and severe sores, scabs and deep scars all over the body. The condition has a death rate of 1 percent to 6 percent.

The C.D.C. doesn't recommend that individuals with eczema or a history of the skin disorder, also known as atopic dermatitis, but vaccinated unless necessary. Given the rise in the number of people who suffer from eczema, up to one-half of the US population may be ineligible for routine smallpox vaccination. The Food and Drug Administration and the National Institutes of Health is negotiating with a private company to produce a smallpox vaccine using a weaker form of vaccinia that would be safer for people at risk from the current vaccine.

How are complications treated?

Two treatments exist for severe side effects:

• Vaccinia immune globulin: This is derived from the blood of donors vaccinated with smallpox vaccine.

• Cidofovir: This is an antiviral drug marketed as Vistide.

At the moment, there’s only enough globulin for about 600 patients, the number of serious complications expected from vaccinating five million people, but many have expressed doubts about the adequacy of this number of doses.

Would vaccination be mandatory?

Vaccination is voluntary even if someone has definitely been exposed to the smallpox virus. Anyone who has been exposed and refuses vaccination may be involuntarily quarantined for up to 18 days. 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 the end of the year. The vaccine is only available through the C.D.C. Mass vaccination would require enormous resources. Clinics would take over schools, warehouses and stadiums. Workers would be needed to give injections, screen patients, get consent forms and control crowds. Isolated hospital wards would be used for smallpox victims and those suffering complications, and to quarantine those who refuse vaccinations.

What would a vaccination program for the entire US cost?

The C.D.C. estimates $5 to $10 a patient, covering the screening and injection only.

CDC Smallpox Website; updated September 23, 2002

Journal of Allergy and Clinical Immunology September 2002;110:357-365