Vaccine efficacy, and immunity affecting transmission.

New candidate malaria vaccines being developed experimentally can only be assessed properly after they have been through the full range of clinical trials and extended follow-up. Defining vaccine efficacy, especially that of multi-component vaccines, is difficult, in particular because the outcome is determined by finely defined epitope specificity. Immune responses can have contrasting effects, some protective, some promoting infection, but concerns that interventions that reduce transmission from a high to a lower intensity will lead to greater risks of severe malaria may not have taken into account all of the many factors that determine the outcome of infection.