Conclusions and Future Directions
Unquestionably, the development of an effective, protective schistosomiasis vaccine would be of immense public health importance. This vaccine can be administered to children in order to prevent severe infection in the following years of high risk (3–12 y of age). This age group of children and young adolescents correspond to those ages in which contact with infected water is maximal. Booster doses of schistosomiasis vaccine may not be necessary since subsequent exposure to infective larvae could provide continuous re-stimulation to immunity. Such a vaccine would greatly reduce the need for logistically difficult and expensive drug-based programs and will save millions of lives. There is a very good likelihood of having a starter vaccine by the next decade. It is also important to realize that the most appropriate clinical endpoint for vaccine efficacy is reduction in morbidity associated with schistosomiasis. To this effect, emphasis should also be placed on exploring the therapeutic potential of antigens in addition to the conventional prophylactic and antifecundity efficacy estimations. Obviously, further research is required on the development of novel adjuvant vehicles as well as cocktail vaccine formulations to enhance protection levels with the eventual aim of 100% worm reduction. Furthermore, it would be sagacious if partly characterized antigens are not rushed pre-maturely into clinical trials without first testing their prophylactic and therapeutic potential in both rodent and nonhuman primate systems. There are several reasons for this cautious approach. For example, Lebens et al.18 have pointed out, and we concur, that some of the proposed vaccination strategies based on studies performed only in the mouse model could have undesirable effects in some individuals if taken to human clinical trials. This is partly because of a developing paradigm which suggests that mechanisms of protection in the permissive mouse model of schistosomiasis cannot completely be generalized to human protection. This belief is based on the failure to know exactly whether a successful schistosome vaccine should induce Th1 or Th2 or both responses that will result in acceptable protection/resistance. Overall, while schistosomiasis represents a major public health burden in areas of the world least-equipped to shoulder it, the future is bright for the development of a vaccine to combat this disease, but more funding and resources should be dedicated to these efforts.