Greek mythology tells the story of Pandora, who lived with her husband, Epimetheus, in a time when the world was free of evils and plagues. One day, Pandora discovered a beautiful box with a sign that said, “Do not open!” But Pandora's curiosity got the better of her. She lifted the lid, and all the troubles and plagues of the world immediately escaped. Of all the imaginary terrors that could have come out of Pandora's box, it is hard to imagine that any could be more evil than the very real orthopoxvirus smallpox.

From 1850 to its eradication in 1979, almost 1 billion people worldwide succumbed to smallpox infection1,2. After 11 September 2001, fears of deliberate infection caused the world to look at smallpox in a new light. It remains possible that stocks of the virus exist outside of their sanctioned deep-freezes in Russia and the US1,2,3,4. Smallpox is a serious terror threat: it can be produced in large quantities, is stable enough for storage and transportation, has a 30% mortality rate in exposed, nonvaccinated individuals and has been known to spread from a single infected individual to 10–20 or more people2,5.

Several vaccine-related issues need to be addressed to ensure public safety. These include the need for a modern alternative to the live animal–produced stock and to determine immune correlates relevant for the twenty-first century in order to test new, safer vaccine candidates. In this issue, Hammarlund et al.6 and Weltzin et al.7 move us considerably closer to these goals. Hammarlund et al. provide evidence that vaccine-induced immunity persists for many years and is, in some fashion, lifelong. Weltzin et al. reveal a new tissue culture method for producing smallpox vaccine that bypasses the methodology of the twentieth century, which required scraping the hides of cows infected with vaccinia virus (Fig. 1), an attenuated orthopox virus that generates cross-protective immunity against smallpox in humans.

Figure 1: Colored transmission electron micrograph of a section through vaccinia virus particles.
figure 1

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The genetic cores (purple) are covered by membrane layers (green and yellow) taken from the host cell in which the virus replicated. This virus is the basis for the smallpox vaccine used in the twentieth century and investigated by Hammerlund et al. Weltzin et al. introduce a new vaccine using vaccinia adapted to tissue culture.

Millions worldwide, and about 90% of individuals in the United States over the age of 35 (half the country's population), were vaccinated before the end of the mass vaccination campaigns. The status of their immunity against smallpox has been under debate4,8,9,10,11. Hammarlund et al. measured T-cell immunity against vaccinia virus in 306 vaccinees, up to 75 years after their last vaccination. Within the first 7 years after vaccination, CD4+ and CD8+ T-cell responses remained high and then declined slowly over decades. The decline in CD4+ T cells, which facilitate immune expansion, occurred more slowly than the decline in CD8+ T cells, which eradicate viral factories. Yet even between 41 and 75 years after vaccination, most vaccinees showed some CD4+, and some had CD8+ T-cell immunity. When they examined humoral immunity, Hammarlund et al. found that most subjects maintained stable antibody responses for up to 75 years after vaccination, suggesting essentially lifelong immunity. This conclusion dovetails with previous studies suggesting that antibody responses that neutralize the virus are associated with effective vaccination. In addition, there seems to be little immunological benefit to multiple vaccine boosts given early, suggesting that the first immunization 'protects' against subsequent vaccination.

The consequences of the waning of cellular immune responses observed by Hammarlund et al. require further study. Is the decrease in CD8+ T-cell responses an early indication of vaccine failure, or might CD4+ T-cell help permit a rapid expansion of CD8+ immunity upon challenge and infection? Studying the usefulness of additional vaccination for people later in life and the expansion of their T-cell response is important, as is studying the effect of decreased T-cell immunity in the presence of persistent antibody in nonhuman primate challenge models. Perhaps heterologous prime-boost strategies, which would not be antibody limited, might be beneficial12. The persistent immune responses observed by Hammarlund et al. suggest that side effects of vaccination, such as eczema vaccinatum13 should occur infrequently in revaccinated individuals. The study also suggests ways to determine correlates of protection in the absence of true smallpox challenge.

While the study by Hammarlund et al. indicates that most vaccinated individuals may retain some immunity against smallpox infection, Weltzin et al. introduce a first-generation replacement for those without immunity. This is an important accomplishment because the current vaccine stocks, even if diluted, would probably not fulfill the demands of unvaccinated individuals in the United States, let alone worldwide5,8,9.

Weltzin et al. adapted the existing Dryvax vaccine, which is derived from the crossprotective vaccinia virus, to a human cell line for production in tissue culture. After animal studies, they chose one clone as the most promising vaccine candidate. In a small clinical study in humans, 100% subjects vaccinated with the new vaccine (ACAM-100) vs. 97% of Dryvax-vaccinated subjects exhibited the hallmark of vaccine take, a significant cutaneous reaction at the site of scarification. Overall, ACAM-100 induced a robust immune response, although there were differences between the two vaccines. For example Dryvax induced higher antibody titers while ACAM-100 vaccination seemed to result in stronger CD4 T cell responses. The vaccines had similar safety profiles, with each participant experiencing at least one mild to moderate adverse event.

Unfortunately, Weltzin et al. did not use the same assays as those used in the longitudinal studies of Hammarlund et al. In the future, such comparisons would increase confidence in this new vaccine. Challenge studies in nonhuman primate models are also warranted.

The safety profile of ACAM1000 seems similar to, and perhaps slightly better than, that of Dryvax. However, the investigators still observed a low rate of lethality in suckling mouse studies, as well as the induction of encephalitis in nonhuman primates, as is the case with Dryvax. In addition, the issue of cardiac toxicity, which has recently plagued vaccination efforts, needs to be clinically evaluated14. In fact, the mass vaccination of health-care workers and law-enforcement personnel has recently been put on hold because of safety issues.

Until proven safe, only when faced with the threat of a smallpox outbreak should such a vaccine be considered for mass vaccination campaigns. The ACAM1000 vaccine, rather than being a truly new approach, is a modern production strategy for an old vaccine. The goal of a new safe and effective smallpox vaccine has not yet been attained15,16, but a suitable stopgap seems to be in place.

These two studies provide new technology as well as a suitable benchmark for the development of the next generation of vaccines. However, the reagents necessary for detailed and defined analyses of humoral and cellular responses in this effort are still lacking. A central repository for reagents to study agents of bioterrorism will significantly aid this effort. In addition, pharmaceutical approaches that could be useful in treating smallpox infection should be moved forward to testing in nonhuman primate studies as rapidly as possible. Such therapeutic agents will further alleviate vaccine dependence and provide an additional cushion of safety.