Vaccinia virus is a live poxvirus vaccine that induces strong crossprotection against smallpox for at least 5 years and partial protection for 10 years or more. Historically, the vaccine has been effective in preventing smallpox infection in 95% of those vaccinated.
The vaccine is administered by dermal scarification (bifurcated or two-pronged needle that is dipped into the vaccine solution). The needle is used to prick the skin a number of times in a few seconds. The pricking is not deep, but it will cause a sore spot and one or two droplets of blood to form. The vaccine usually is given in the upper arm. Appearance of a vesicle or pustule within several days is indication of a successful vaccination. Vaccinia-immune human globulin at a dose of 0.3 mg/kg body weight provides >70% protection against naturally occurring smallpox if given during the early incubation period.
Administration immediately after or within the first 24 hours of exposure would provide the highest level of protection, especially in unvaccinated persons. The antiviral drug, n-methylisatin ß-thiosemicarbazone (Marboran®) afforded protection in some early trials, but not others, possibly because of noncompliance due to unpleasant gastrointestinal side effects.
Routine smallpox vaccination among the American public stopped in 1972 after the disease was eradicated in the United States. Due to the possible threat of bioterrorism, according to the Center for Disease Control (CDC), as of 2007, the U.S. government has enough vaccine to vaccinate every person in the United States in the event of a smallpox emergency.
Most experience soreness or redness in the location of vaccination. The glands in the armpits may become large and sore. The vaccinated person may run a low fever. One out of 3 people may feel bad enough to miss work, school, or recreational activity or have trouble sleeping.
In the past, about 1,000 people for every 1 million people vaccinated for the first time experienced reactions that, while not life-threatening, were serious. These reactions may require medical attention:
- A vaccinia rash or outbreak of sores limited to one area. This is an accidental spreading of the vaccinia virus caused by touching the vaccination site and then touching another part of the body or another person. It usually occurs on the genitals or face, including the eyes, where it can damage sight or lead to blindness. Washing hands with soap and water after touching the vaccine site will help prevent this (inadvertent inoculation).
- A widespread vaccinia rash. The virus spreads from the vaccination site through the blood. Sores break out on parts of the body away from the vaccination site (generalized vaccinia).
- A toxic or allergic rash in response to the vaccine that can take various forms (erythema multiforme).
Rarely, people have had very bad reactions to the vaccine. In the past, between 14 and 52 people per 1 million people vaccinated for the first time experienced potentially life-threatening reactions. These reactions require immediate medical attention:
- Eczema vaccinatum. Serious skin rashes caused by widespread infection of the skin in people with skin conditions such as eczema or atopic dermatitis.
- Progressive vaccinia (or vaccinia necrosum). Ongoing infection of skin with tissue destruction frequently leading to death.
- Postvaccinal encephalitis. Inflammation of the brain.
People with certain medical conditions-including people with weakened immune systems or certain skin conditions-are more likely to have these reactions and should not get the smallpox vaccine unless they have been exposed to smallpox. Based on past experience, it is estimated that between 1 and 2 people out of every 1 million people vaccinated may die as a result of life-threatening reactions to the vaccine.
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