The Epidemiology of Variola Major

            http://austinpublishinggroup.com/infectious-diseases/fulltext/ajid-v3-id1023.php

Good studies of the epidemiology of smallpox (Variola major) were conducted in Asia, Africa, and Europe in the late 1960’s and early 1970’s. These studies showed that transmission is fairly slow and generally to susceptible persons who had prolonged face-to-face contact with patients, often as caregivers. Patients are not infectious during the violent prostrating prodrome caused by intense viremia. The rare exceptions to this pattern include true airborne spread, and spread by aerosols created by shaking out heavily infected bedding.Vaccination of contacts within two or three days after exposure generally protects them and aborts the disease. Thus contacts must be identified quickly and vaccinated with good vaccine and vigorous technique.

These epidemiologic facts led to the development of the surveillance and containment methodology, which became the main tool for the eradication program. This consisted of active search for patients, effective patient isolation, identification of their close contacts, vaccination of contacts, supervision of the contacts to immediately isolate them if they developed a fever, and if time and staff permitted identification of the contacts of contacts, the “second ring”. In the unlikely occurrence of a bioterrorist attack employing smallpox, surveillance and containment methods should be the mainstay of control efforts.

In the late 1960’s, the Smallpox Eradication Program switched from a strategy of mass vaccination to intensive surveillance and containment of outbreaks. This change brought about rapid success of the eradication program. The switch was based on studies of the epidemiology of smallpox in West Africa, India, East and West Pakistan, Bangladesh  and Europe conducted in the late 1960’s and early 1970’s. These studies showed that Variola major was less infectious than previously believed, that its spread was usually quite limited, and that vaccination of the immediate contacts of patients rapidly eliminated the disease. This paper reviews these findings and discusses our current understanding of the transmission and spread of the disease.

Variola major is spread from person to person by direct, usually prolonged, face to face personal contact, during which a non-immune individual inhales virus particles from the exhalations of the infected patient. There is no animal host of variola virus, and no one can become an asymptomatic carrier. Patients are not infectious during the prodrome despite an intense viremia. Patients become infectious from the appearance of the rash on the skin and the pharynx, usually two or three days after the onset of the prodrome. The soft tissue in the back of throat has no covering like the squamous layer of the skin, so the virus is shed into the nasopharynx before it is shed from the skin lesions. Most spread is generally to bedside care givers, since most virus is carried in large droplet nuclei which rarely travel more than 6 feet from the patient. Most patients can remember being at the bedside of an obviously ill individual.