Determining the mechanisms of protection is the function, and the importance, of the laboratory trial. There are certain elements, however, relating to the success of a vaccine that are not addressed in the laboratory. Here, for the first time, we describe an in vivo challenge model for testing vaccine efficacy in a natural setting. There are distinct problems associated with testing in an open environment (e.g., determining what constitutes an appropriate control group), but they are the same problems that will need to be addressed when assessing a human vaccine in the field. Further, this model addresses the effect of a vaccine on (i) a nonvaccinated cohort living in close proximity to vaccinated individuals and (ii) the acquisition of reinfection immunity. We are cognizant of the fact that both the vaccine data and the information regarding the effect of herd immunity on vaccine outcome presented here are based on small sample sizes. An investigation of this type, however, should help us to develop an appropriate means of assessment of field trials for a human vaccine.
In this study, two effects of using a DNA vaccine in canary populations in the Baltimore Zoo were observed. The survival from death due to malaria was enhanced by the vaccine during the first year. Based on the results of ELISAs, all control birds were exposed to sporozoites during the trial, and hence the vaccine appeared to be responsible for protection. Also supporting this conclusion is the fact that nonvaccinated birds in site B died in significantly greater numbers than vaccinated birds in site A. The only consistent difference between the sites of which we are aware is the vaccine.
Vaccination had another surprising effect. It seemed to interfere with the acquisition of reinfection immunity because in the follow-up year all birds that died were ones that had previously been vaccinated, while those that had acquired immunity as a result of exposure were fully protected. The relevance of these observations is discussed below.
DNA vaccines have been shown to be an excellent way to induce a class I-dependent cellular type of immune response. Because avian red cells are nucleated and express class I molecules, it is reasonable to assume that our DNA vaccine induced effective anti-CSP CD8+
-T-cell responses in vaccinated birds, leading to either a total elimination or significant reduction in blood-stage infection. Further, in murine malarias, DNA vaccine-induced immunity against preerythrocytic stage parasites is primarily mediated by class I-dependent CD8+
-T-cell responses (8
We find that CSP is expressed in both preerythrocytic- and erythrocytic-stage parasites in P. relictum
(K. C. Grim, J. Li, and T. F. McCutchan, unpublished data) and, therefore, anti-CSP immune responses could lead to either sterilizing immunity or a significant reduction in parasite burden. In the avian, there are other important differences in lymphatic organs such as the bursa of Fabricus. In this context, it is important to note that some of the most dramatic results from the use of DNA vaccines against infectious diseases have been achieved in avian models (28
How the DNA vaccine was effective in protecting its neighbors remains unclear. Two factors that are known to be involved with malaria control are location effects and herd immunity. It is possible that a high degree of protection in nonvaccinated cage mates is the result of herd immunity. We speculate that vaccination-induced immune responses, both against sporozoites and blood-stage parasites, cause a significant reduction in parasite burden. With regard to avian malarias, this could be a factor in reducing inoculation rates within the time frame of the trial and lead to lower mortality rates. The effect of herd immunity on improved protection has been described in relationship to vaccine trials against Lyme disease (5
). Herd immunity has been shown to play a critical role in the outcome of clinical trials of bed nets impregnated with insecticide in sub-Saharan Africa. The use of bed nets led to a significant reduction in child mortality in both the children that used them and their neighbors that did not (4
Location is also a factor in infection and could lead to an alternate interpretation of results. For example, there is a marked clustering of symptomatic malaria associated with particular sites, usually households (hence, references to a malaria house). Therefore, even though we have shown that the cage mates of the vaccinated individuals were challenged during the trial year, we acknowledge that symptomatic disease and exposure to sporozoites are different things. It is possible that symptomatic disease may not have occurred during the trial year in site A. This is possible despite the fact that normal mortality rates were seen there in both the preceding and following years.
The follow-up year indicated that even though every bird was challenged, the vaccinated birds and their cage mates had not acquired lasting immunity to disease in their first year of exposure. It appears that the acquisition of natural immunity was affected by the introduction of a vaccine that may have caused a total elimination or a significant reduction in parasite burden. This observation suggests that introduction of an effective vaccine in areas of endemicity could impair host immunity and worsen the malaria situation in the following transmission season. In conducting human trials, it would therefore be prudent to carefully monitor the trial subjects and their families and neighbors for an extended time.
As we point out above, there are alternative explanations of prevention data, as one might expect when subjects are introduced into a natural environment. Each alternative, however, suggests experiments to resolve the conflict with the avian model. No other model offers the opportunity to investigate these aspects of interplay between approaches to malaria prevention and the environment.