Vaccinia

Vaccinia is a large, complex virus that belongs to the poxvirus family of viruses. Because vaccinia is closely related to but less dangerous than the smallpox virus, it can be administered as a vaccine to protect against smallpox, monkeypox, and other infections arising from viruses of the Orthopox genus. As a result, vaccinia is one of the most studied viruses in the world and among the most widely utilized for immunization purposes. In fact, the eradication of smallpox was a direct result of inoculation through vaccinia. The vaccinia-based smallpox vaccine is particularly effective because, unlike many other vaccines, it contains live samples of the vaccinia virus. This means that inoculation with vaccinia may result in a mild skin infection in the area where the shot is administered. In addition to its use as a vaccine, vaccinia is also commonly studied as a potential tool for gene therapy and genetic engineering.

Background

The discovery of the vaccinia virus is directly tied to the history of smallpox. A potentially deadly disease caused by the variola virus that typically produces pock-like lesions on the skin and other organs, smallpox probably began infecting humans around 10,000 years ago. First widely known in China during the eleventh century BCE, smallpox spread to Europe sometime between the fifth and seventh centuries BCE. By the seventeenth and eighteenth centuries CE, smallpox epidemics were a common occurrence across Europe. Smallpox also eventually made its way to the Americas, likely due to early Spanish explorers. During its most widespread period, smallpox could be found everywhere around the world outside of Australia and a number of isolated islands.

While attempts to control the spread of smallpox were undertaken as early as the sixth century BCE, it was not until the late eighteenth century that an effective means of doing so was actually developed. The crucial breakthrough was made by British physician Edward Jenner in the 1790s. While studying smallpox, Jenner found that milkmaids who had been exposed to cowpox appeared to become immune to smallpox. His examination of this phenomenon ultimately led him to discover the vaccinia virus, which he subsequently used to create a vaccine with which he successfully inoculated nine-year-old boy against smallpox in 1796. In doing so, Jenner developed the world’s first vaccine.

The smallpox vaccine was remarkably effective. After its early adoption in the nineteenth and early twentieth centuries demonstrated its ability to control the spread of smallpox, the World Health Organization (WHO) set its sights on the total eradication of the disease starting in 1967. Only a decade later, the last endemic case of smallpox was reported in Somalia. The disease was officially declared eradicated in 1980. Since that time, smallpox has largely existed only in laboratories. As a result, routine smallpox vaccination was suspended in the United States in 1972, though inoculation via vaccinia is still occasionally provided to some individuals under certain circumstances. Lab workers who come in contact with the variola virus, for example, may be given smallpox vaccines due to the increased risk of infection they face.

Overview

Vaccinia is a virus that may produce mild symptoms in the event of an infection. It is most commonly encountered in the course of its use as a smallpox vaccine. As a result, localized vaccinia infections typically occur around the inoculation site when this vaccine is administered. Such infections rarely have serious consequences.

The origins of the vaccinia virus remain unclear even more than a century after it was first isolated. Some scientists believe vaccinia could be the result of genetic recombination. Others suggest that it could have evolved from variola or cowpox. Still others think vaccinia might be the living representative of an extinct virus. The challenge of determining the virus’ origins is further complicated by the fact that different strains of vaccinia were independently developed in various European and Asian cities in order to combat smallpox. One emerging hypothesis as to vaccinia’s origins is that the virus may be a hybrid of both variola and cowpox.

The vast majority of vaccinia infections are the direct result of smallpox vaccination. Under normal circumstances, an itchy red lesion develops at the inoculation site approximately three to four days after the vaccine is administered. Over the course of around three weeks, the lesion grows into a blister, fills with pus, drains, and forms a scab that eventually falls off and leaves a small scar. The localized vaccinia infection caused by smallpox inoculation can spread if the patient touches the vaccination site before it properly heals or touches bandages or clothing contaminated with the live virus. Individuals who have not received the smallpox vaccine can develop a vaccinia infection if they come into close physical contact with someone who has recently been vaccinated.

The symptoms of vaccinia infection are a milder version of those associated with smallpox. Classical signs of infection include the appearance of red spots on the skin, fever, and general common cold-like symptoms. More serious symptoms are rare and usually occur only in individuals who are immunocompromised or have pre-existing skin conditions. Possible serious symptoms of vaccinia infection include conditions such as eczema vaccinatum, progressive vaccinia, post-vaccination encephalitis, or myocarditis or pericarditis. Eczema vaccinatum is a severe skin rash that can develop in patients who already have eczema or other similar skin conditions. Progressive vaccinia occurs when the vaccinia virus spreads uncontrolled to other nearby tissues, which can lead to tissue death. Post-vaccination encephalitis results when a vaccinia infection spreads to the brain following an inoculation. Myocarditis or pericarditis are forms of heart inflammation that may occur when the body over-responds to vaccination with vaccinia.

Since most vaccinia infections produce only mild symptoms that resolve on their own, treatment is usually unnecessary. More serious infections may require medical intervention. In the event of eczema vaccinatum or progressive vaccinia, for example, patients may be treated with vaccinia immune globulin (VIG), a hyperimmune globulin specifically developed for the treatment of severe vaccinia infections.

Bibliography

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