**8. Concluding remarks**

**7. Transmission blocking vaccines**

348 Towards Malaria Elimination - A Leap Forward

might help to define correlate of protections for TBV development.

Transmission blocking vaccines (TBV) are designed to prevent or reduce the development of the sporogonic stage inside the mosquito host. This leads to a reduction in the numbers of infected mosquitoes and hence lesser malaria transmission in the population. As such, experimental clinical trials cannot provide straightforward answers of vaccine efficacy since the effect of such vaccine is at the population level and not at individual level. Assessment of TBV efficacy is done using *in vitro* assays and the membrane-feeding assay. Mosquitoes are fed *in vitro* with *Plasmodium* gametocytes mixed with serums from immunized individuals and the level of sporogonic development is assessed by counting the oocysts in dissected mosquito midguts [104] or, more rarely, the sporozoites in the mosquito salivary glands. Although this assay has been instrumental in identifying target antigens, it remains to be seen whether it

Currently, none of the TBV has progressed to clinical trials in the field. As the principal outcome of TBV is to reduce the number of infected mosquitoes at the population level, methods for assessing their number in field conditions should be implemented in a timely manner. There are, to date, no standardized methods to estimate the number of infected mosquitoes in the field and estimation would require large sampling size. Moreover, it seems that infected mosquitoes can cluster in discrete locations [105], thus requiring extensive studies on the distribution of infected mosquitos before and after vaccine trials. Since the number of infected mosquitoes depend on the number of circulating gametocytes [106], defining the number of gametocyte carrier prior to vaccine implementation is also a pre-requisite. In addition, defining transmission intensity of the vaccine site is important since it may influence the outcome of the vaccination. This can be defined by seroepidemiology and geographical information system (GIS) applications. For the latter, GPS mapping of mosquitoes and infected humans needs be done. It must be noted that one major limitation of these trials is that they assume that the human and mosquito populations tested are not mobile, which is often not true. An influx of infected individuals can modify the outcome by creating new reservoirs, and an influx of external infected mosquitoes would maintain transmission. It has been suggested that TBV can be tested with accuracy only in enclaved locations such as islands. Ultimately, the main expected outcome is that TBV will reduce transmission and thus reduce morbidity and mortality. The effect of such vaccine is at the population level, a large and costly cohort will be needed to be assessed over a long period of time. However, recent advances in modeling might facilitate TBV assessment by identifying end-point measures, which may serve as correlates of protection [107, 108]. Different end-point measures have been developed to assess TBV efficacy. For TBV that targets gametocytes, numbers of gametocytes and duration of gametocytes are important measures since it is expected that reduction in the number of gametocyte-carriers (reservoir) will decrease transmission. Microscopic determination of gametocytes on Giemsa-stained smears has long been used but they must be complemented with PCR methods since they have shown to underestimate gametocyte load [109]. Gametocyte infectivity to mosquitoes can be measured with the membrane-feeding assay mentioned earlier or with feeding of mosquitoes directly on the skin of gametocyte carriers, which reproduces the natural situation. An honest correlation Here we discussed the different types of malaria vaccines and the different ways to access the vaccine efficacy. We also highlighted the limitations involved and the difficulties encountered by researchers aiming to develop an efficacious vaccine against a complex parasite such as *Plasmodium*. Despite decades of research efforts in vaccine development, no efficient malaria vaccine (i.e. with an efficacy >50%) has been developed. The most clinically-advanced RTS,S, which has been tested in Phase III, conferred at best 30–40% protection against clinical malaria [112]. Modeling studies have been proposed that, together with other malaria interventions, RTS,S vaccination may reduce the incidence of clinical malaria and deaths in many sub-Saharan African countries [113, 114]. Thus, this has led to the approval of licensure for the RTS,S vaccines by the European Union. However, the World Health Organization has not recommended its use in the extended program of immunization for children due to its discouraging vaccine efficacy data. Thus, the future of this vaccine for mass deployment remains uncertain. One of the major reasons of the limited efficacy of RTS,S vaccines and the discontinuation of various other vaccine development efforts is certainly due to antigen polymorphism [83]. In addition to antigen polymorphism, the malaria parasite utilizes many other immune escape mechanisms [115], which have severely hampered the development of malaria vaccines. With the renewed interest in malaria eradication, the development of an effective malaria vaccine is high on the agenda. Diverse strategies are being proposed to develop better vaccines: identification of new vaccine candidate [116], combinations of different antigens targeting the same stage or different stages [117]; new delivery systems and prime-boost strategies using different modalities [118]; and new adjuvants to induce stronger and longer lasting efficient immune responses [119–122]. However, for all vaccine types described, the absence of validated surrogates of protection to help select and prioritize different vaccine formulations is a major roadblock, which should be given priority to accelerate vaccine testing.
