to 5 years, and some level of immunity, while diminishing over time, may persist for up to 10 years, and perhaps even longer (CDC, 2001; Cohen, 2001; WHO, 2001; Eichner, 2003). Current research is still in its early stages and takes place in the absence of actual smallpox disease, relying instead on three surrogate measures of immunity: neutralizing antibody, cellular immunity, and skin reactions. There is some evidence that significant immunity may be maintained beyond five to ten years after vaccination. Crotty and colleagues (2003) found that smallpox-vaccine–specific memory B cells may persist for longer than 50 years after immunization. Also, Hammarlund and colleagues (2003) found that more than 90 percent of volunteers vaccinated 25-75 years ago exhibited stable levels of vaccinia-specific antibody, and persisting, though diminishing antiviral T-cell response. There is little agreement whether these findings can be interpreted to mean that individuals vaccinated before 1972 would have any significant level of protection against smallpox (Roos, 2003). Additional research is needed to shed more light on this complex matter.
Dryvax vaccine is a highly stable, live-virus vaccine containing the vaccinia virus, another orthopoxvirus. Vaccinia’s origins are unclear, as it differs from Jenner’s “variolae vaccinae,” but vaccinia has been widely studied, and much of what is known about orthopox viruses was first learned from this species (Fenner et al., 1988).
Immunization with vaccinia-based vaccines involves inoculation of the skin using a bifurcated needle that holds a dose of the vaccine (a small drop) in its fork, and that is first used to release the liquid on the skin and then, held perpendicular to the skin, to rapidly and vigorously puncture the skin in an area of about 5 mm diameter, making a trace of blood appear (CDC, 2003c).1 Reaction to the vaccine, or “vaccine take,” can be evaluated based on the appearance of the skin lesion that develops after vaccination. There are two types of reactions: major and equivocal. A major reaction, proof of successful vaccination, consists of “a pustular lesion or an area of definite induration or congestion surrounding a central lesion, which might be a scab or an ulcer” (CDC, 2003d). The size of lesions peaks between days 8 and 12, and the infection is sometimes accompanied by mild fever and malaise. Three weeks after vaccination, the scab falls off, leaving a small