The RTS,S/AS01 malaria vaccine is a recombinant protein vaccine in which the fusion protein RTS (containing parts of the circumsporozoite protein (CSP) of Plasmodium falciparum fused to hepatitis B surface antigen (HBsAg)) is co-expressed in yeast together with free HBsAg (S) to form a virus like particle (RTS,S); it is given with the powerful adjuvant AS01. RTS,S/AS01 induces a strong antibody response to the P. falciparum CSP and high titres of anti-CSP antibody are associated with protection. Following a long process of development, a phase 3 study of RTS,S/AS01 conducted in 15,439 children in 7 countries in Africa showed that three doses of RTS,S/AS01 given with a one month interval between doses, followed by a fourth dose 18 months post dose 3, gave 36.5 % [95% CI 31,41%] protection against clinical attacks of malaria when given to young children aged 5-17 months who were followed for 48 months; efficacy was less when given to infants at the age of 6-12 weeks. RTS,S/AS01 provides a high level of protection during the first three months after vaccination, modelled to be about 70% in the phase 3 trial, a level of initial efficacy similar to that observed in an earlier phase 2 trial in Gambian adults. However, efficacy wanes progressively over the following months. A subsequent dose given 18 months after the primary series restores some but not all of the efficacy seen immediately after the primary series. In July 2015, the European Medicines Agency reviewed efficacy and safety data on RTS.S/AS01 and concluded that the risk benefit balance favoured the vaccine and gave a positive opinion on its use in children aged 6 weeks to 17 months. The World Health Organization (WHO) Strategic Advisory Group of Experts (SAGE) committee reviewed the vaccine's efficacy and safety in October 2015 and made a number of recommendations on its further evaluation. These included the pilot implementation of RTS,S/AS01 in children aged 5-17 months in 3-5 settings with moderate-to-high malaria transmission intensity, with a preference for areas where SMC is not being delivered, and evaluation of alternative approaches to deployment of the vaccine. Recent evidence from challenge studies conducted in American adult volunteers suggests that a higher level of protection can be obtained when the third dose of the priming schedule is reduced to one fifth of the usual amount and delayed until approximately 6 months post dose 2, and when a reduced dose is used for boosting. In these studies, a vaccine efficacy of 86% was achieved three weeks following priming and 90% efficacy following boosting with a fractional dose. This encouraging result is now being followed in further studies.