Historically, individuals in the US were vaccinated with a product such as Dryvax®, which contains the virus vaccinia in the same family as smallpox. This virus could promote immunity to smallpox, but not produce the disease itself. Although effective, these vaccines are not safe to use in people with atopic dermatitis (eczema, allergic immune response to allergens), children less than 1 year of age, and people with a compromised immune system, occurring in certain diseases (HIV positive individuals and AIDS), and following treatment with certain types of drugs. It is important to find a safe vaccine that can be used to protect people who cannot receive routine vaccinia-based smallpox vaccine.