be able to rely on natural boosting after primary immunization with a transmission-blocking vaccine. The use of novel adjuvants, carrier proteins, and slow-release vaccine formulations may overcome this problem.

Enhancement of Infectivity At least with P. vivax and the monkey malaria parasite P. cynomolgi, many naturally infected individuals develop antibodies that inhibit parasite development in mosquitoes (Mendis et al., 1987). Early in the course of infection and very late after infection, when the levels of transmission-blocking antibodies are low, parasite development and transmission may be enhanced (Peiris et al., 1988). Enhancement is also observed with low concentrations of some inhibitory monoclonal antibodies, although not with antibodies to Pfs25 and its homologous antigens in other malaria parasites. Although this may superficially suggest that transmission-blocking vaccines may actually enhance transmission as vaccine-induced antibody levels decline, it seems unlikely that vaccination would exacerbate the enhancement that occurs naturally (and may be an important factor sustaining malaria transmission in some areas).

Cost and Acceptability of an Altruistic Vaccine Because transmission-blocking vaccines will not protect individuals, they would not be used alone in nonimmune visitors to malaria-endemic areas. Consequently, there is even less commercial incentive to develop these vaccines than pre-erythrocytic or blood-stage vaccines. Vaccines that protect populations but not individuals might also have to be safer and have fewer side effects than vaccines that provided protection to the vaccinated individual. It is likely that transmission-blocking vaccines will be used primarily in conjunction with pre-erythrocytic and blood-stage vaccines, in which case they may have the added benefit of reducing the transmission of vaccine-resistant parasite strains.

Antimosquito Vaccines

Of the many methods proposed and tested for controlling mosquito populations, one of the most unusual is the use of antimosquito vaccines. In this approach, humans would be vaccinated with mosquito proteins critical for insect viability. The resulting human antimosquito antibodies are ingested in the mosquito's blood meal. The goal is to reduce parasite transmission by reducing the vitality or vector capability of the mosquito. An antimosquito vaccine that reduced the adult mosquito 's life span, so that parasite development would be interrupted, might have a significant effect on transmission.

Early studies reported that mortality of Anopheles stephensi was increased when these mosquitoes fed on rabbits that had been immunized with mos-



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