The most ambitious clinical trial of a malaria vaccine has shown some effectiveness in children over an 18-month period. While its efficacy is modest, it is nonetheless a significant advance in the long struggle to control a disease that kills some 600,000 people a year, mostly children under the age of 5.

The trial, conducted at 11 research centers in seven African countries, involved more than 15,000 patients who took a new malaria vaccine developed by GlaxoSmithKline or a non-malarial vaccine given to control groups. Young children who were 5 months to 17 months old when first vaccinated with the Glaxo vaccine had 46 percent fewer cases of clinical malaria than the control groups. Infants who were 6 weeks to 12 weeks old when first vaccinated had 27 percent fewer cases.

Glaxo spent more than $350 million over 25 years to develop the vaccine for military personnel and travelers and expects to invest an additional $260 million to complete development. But Glaxo was reluctant to pay for pediatric trials in impoverished nations on its own, so the Bill and Melinda Gates Foundation provided $200 million through the nonprofit PATH Malaria Vaccine Initiative to drive development and testing over the finish line.

If the results hold up after further follow-up, the vaccine will be the first ever shown to be effective on a large scale against a disease-causing parasite, an organism that is much harder to neutralize than viruses or bacteria. The vaccine attacks the parasite at its earliest stages, before it can multiply in the liver and re-enter the bloodstream to infect red blood cells. The trials were conducted in real-world conditions in which a large majority of young children and infants were already protected by sleeping under insecticide-treated bed nets and other prevention measures. The vaccine protected them further.