Smallpox Fact Sheet

What is smallpox?
Smallpox is an often-fatal viral disease caused by the Variola virus.

When was it eradicated?
Smallpox was eradicated from the world in 1977 after a worldwide vaccination program led by the World Health Organization. It was eradicated from the Western Hemisphere in 1971. Smallpox was eradicated with a strategy that relied on surveillance for cases of the disease, isolation of the cases whenever possible and vaccination of contacts. There was no need for mass vaccination of an entire country.

How was it spread?
Smallpox was spread, most often, person to person, when an ill person released virus-infected saliva droplets from their mouth into the air and these were in turn inhaled by a susceptible person in close contact with the ill person (face-to-face contact). The first week of illness was the most infectious.

How did the disease progress?
The ill person first develops the rash throughout the first week of illness, however, the person is still infectious until the rash has resolved (all scabs have fallen off). Virus is also present in the scabs that separate from the skin but these are much less infectious than saliva. Case fatality rates range from 1 to 30 percent, with deaths most often occurring during the first or second week of illness. The incubation period is about 12 days (range: 7 to 17 days) following exposure. Symptoms include high fever, fatigue, and head and backaches, which are followed in 2-3 days by the rash. Lesions in the mouth and throat appear early in the illness and ulcerate to release large amounts of virus in the saliva. The most visible symptom of smallpox is a rash with lesions densest on the face, arms and legs. The lesions are round, dense, and deeply embedded in the skin.

The Risk
Routine vaccination against smallpox stopped in 1972 in the United States and in the late 1970s and early 1980s in the rest of the world. The level of immunity among persons older than 27 in the United States is uncertain, and the duration of immunity has not been well measured.

Prevention
Vaccine against smallpox was a live virus vaccine, made with a related virus called vaccinia virus. It does not contain smallpox virus.

In 1980, the World Health Assembly, made up of all member countries of WHO, recommended that all countries cease vaccination and that all laboratories destroy their stocks of variola (smallpox) virus or transfer them to one of two World Health Organization reference labs. All countries reported compliance.

The United States currently has a limited supply of smallpox vaccine (approximately 15 million doses) available for emergency use, if needed. New methods for the production of additional smallpox vaccine in large quantities are being explored. At this time, no preventive vaccination program is planned.

Smallpox vaccine is very effective. Individuals adequately vaccinated will not contract the disease. The vaccine also can lessen the severity or even prevent illness in people exposed to smallpox if given up to 4 days after exposure.

People with smallpox must avoid contact with unvaccinated individuals in order to prevent transmitting the disease to them.

Treatment
At this time, there is no proven treatment for smallpox. Patients with the illness would be given non-specific supportive therapy as needed (intravenous fluids, medicine to control fever or pain, etc.) and antibiotics for any secondary bacterial infections that occur.

No antivirals have yet proved effective for treating smallpox; however, research is ongoing.

A smallpox outbreak would spread unless checked by vaccination and monitoring of contacts to smallpox patients and isolation of infectious smallpox patients.

Biological Threat
It is not known if the variola virus is kept by any state or terrorist groups for the purpose of bacteriological war or terrorism.

Because this virus is relatively stable (not easily destroyed in the environment) and the infectious dose is small, an aerosol release of variola virus could disseminate widely.

A single suspected case of smallpox would be treated as a health emergency and should be brought to the attention of national officials through local and state health authorities. However, varicella, or chickenpox, which infects millions of children each year, is the disease most frequently confused with smallpox. (Chickenpox lesions are much more superficial and are almost never found on the palms and soles.)

For more information, please contact: Daniel Epstein, Office of Public Information, (202) 974-3459, epsteind@paho.org