Hardest-hit Nigeria is latest African country to provide malaria vaccine to young children / Malaria vaccine rolled out in world's worst-affected country / Sudan launches first malaria vaccine in landmark child health initiative / South Sudan receives its first batch of a new vaccine for malaria from the WHO / Malaria vaccine makers call for more orders to drive down costs / Malaria vaccine is highly effective in young children, study suggests / In Africa, world health officials roll out first routine malaria vaccine