Knocking out just three of the 5000 genes in the P. falciparum genome can prevent sporozoites from maturing and reproducing, so they can’t infect the bloodstream.
They infected mosquitoes with the genetically-engineered sporozoites and let them snack on the volunteers’ forearms. And none of the people in the trial developed malaria, so that’s good!
Each sporozoite produces as many as 30,000 mature parasite cells called merozoites, which spread through the bloodstream, burrow into red blood cells, and cause a full-blown infection.