Background information

Smallpox was eradicated by an international campaign led by the World Health Organization (WHO) in the 1970s. Until recently, there has been little concern about the use of smallpox as a biological weapon. Currently, the only known remaining virus samples are stored in the Centers for Disease Control (CDC) laboratories in Atlanta, Georgia, and in Russia. Since the terrorist attacks on the United States, concern over the possible use of smallpox as a biological weapon has increased. Attributes of smallpox that make it a potentially effective weapon include its highly infectious nature, the high fatality rate, and the population's lack of protection from the disease either via natural immunity or sufficiently recent vaccination.

What is it?

Smallpox is a highly infectious disease caused by the variola virus that is spread from person to person through face-to-face contact. The incubation period is usually between 12 and 14 days and is not accompanied by symptoms. The 2 main forms of the disease are: variola major and variola minor. The more severe form of the disease is variola major, which has a fatality rate of about 30%. The less severe form of the disease is variola minor, with a fatality rate of less than 1%. Because patients experience milder symptoms with variola minor, many are ambulatory and can unknowingly spread the disease to a wider population during the infectious stage. Patients suffering from variola major usually became bedridden early on and remain so throughout the illness.

Around the second week, following the incubation period, there is a sudden onset of flu-like symptoms including fever, headache, back pain, and fatigue. Two or 3 days later, the characteristic rash starts to appear on the face, arms and legs. On about the eighth or ninth day, the skin lesions become postular and eventually become crusty. During the last stages, the scabs separate and fall off.

Transmission of smallpox cannot occur during the incubation period. Thereafter, transmission of smallpox can occur until the last scabs have fallen off, but the risk of infection is much lower at this stage. Transmission can also occur through contaminated clothing and bedding, but infection is less likely. Animals and insects cannot carry the disease.

What is the potential for a smallpox outbreak?

During the WHO eradication campaign in the 1970s, investigations revealed that smallpox could sometimes spread in closed environments via ventilation systems. In terms of natural spread via face-to-face contact, the initial or "index" case has been known to infect up to 5 people and in rare cases more than 12. These historical data are available only from periods when a substantial portion of the population was immune to smallpox either from vaccination or from surviving natural infection. In the absence of natural disease or recent vaccination, the global population is significantly more susceptible. Some experts have estimated that the rate of transmission under present circumstances could be of the order of 10 new infections per infected person.

How long does the smallpox vaccination last?

Persons who have been vaccinated against smallpox usually have immunity for at least 10 years.

Is smallpox vaccination dangerous?

Routine vaccination for smallpox ceased in 1972. Complications from smallpox vaccination are infrequent but may include less severe symptoms - such as treatable rashes, and severe symptoms such as brain inflammation (encephalitis). Hence vaccination is only warranted in persons who have been exposed to the virus or are at risk for exposure.

What is the treatment for smallpox?

There is limited treatment for smallpox once symptoms have started. If smallpox vaccination is given immediately after exposure (within 1 to 4 days), it is possible that illness can be prevented, or the severity of the sickness reduced. In the past the death rate from smallpox was as high as 30%. Complications from smallpox include scarring, secondary bacterial infection of the skin at the lesion sites, arthritis and bone infections, pneumonia, severe bleeding, eye infections, and brain inflammation. Between 65% to 80% of survivors were marked with deep pitted scars (pockmarks), most prominent on the face.

Antibiotics can be given to treat secondary infections.

Any exposed persons need to be isolated. This includes the person who has contracted the disease and all other face-to-face contacts with that person.

Suzette Alvarez 
in association with the MediResource Clinical Team