The article doesn't say but my understanding is this was a 13% reduction in all-cause mortality as in they measured the number of children who died during the time period and that went down by 13%. Not that 87% of malaria related deaths still occurred. A 13% reduction in all-cause mortality is quite large. To know how effective the vaccine is at stopping malaria related deaths you'd need to know what % of those who were dying were dying to malaria. If it was 14% then the vaccine is performing insanely well if it was 50% more work needs to be done.