On Oct. 31, 2022, the National Institutes of Health announced that a clinical trial had found that one dose of an antibody drug safely protected healthy, non-pregnant adults from malaria infection during an intense six-month malaria season in Mali, Africa. The antibody was up to 88.2% effective at preventing infection over a 24-week period, demonstrating for the first time that a monoclonal antibody can prevent malaria infection in an endemic region. These findings were published in The New England Journal of Medicine and presented at the American Society of Tropical Medicine & Hygiene 2022 Annual Meeting in Seattle. 

An estimated 241 million cases of malaria occurred worldwide in 2020, according to the World Health Organization (WHO), resulting in an estimated 627,000 deaths, mostly in children in sub-Saharan Africa. These cases included more than 11 million pregnant women in Africa, resulting in an estimated 819,000 newborns with low birthweight and thus at increased risk for illness and death. 

The only malaria vaccine currently recommended by WHO, called RTS,S (Mosquirix), provides partial protection against clinical malaria during the early years of life when given to children aged 5 to 17 months in four doses over a 20-month period. Other drugs consisting of small chemical compounds that effectively prevent malaria infection are also available for infants and young children as well as travelers. The requirement for frequent dosing of these drugs can limit adherence, however, and the emergence of drug resistance may also limit their usefulness. Thus, there is an urgent need for new, fast-acting, infrequently dosed interventions that safely provide strong protection against malaria infection.

Malaria is caused by Plasmodium parasites, which are transmitted to people through the bite of an infected mosquito. The mosquito injects the parasites in a form called sporozoites into the skin and bloodstream. These travel to the liver, where they mature and multiply. Then the mature parasite spreads throughout the body via the bloodstream to cause illness. P. falciparum is the Plasmodium species most likely to result in severe malaria infections, which, if not promptly treated, may lead to death.

The Phase 2 NIAID-USTTB trial evaluated the safety and efficacy of a one-time, intravenous infusion of a monoclonal antibody called CIS43LS. This antibody was previously shown to neutralize the sporozoites of P. falciparum in the skin and blood before they could infect liver cells. Researchers led by Robert A. Seder, M.D., isolated a naturally occurring form of this antibody from the blood of a volunteer who had received an investigational malaria vaccine, and then modified the antibody to extend the length of time it would remain in the bloodstream. Dr. Seder is the acting chief medical officer and acting associate director of the NIAID Vaccine Research Center (VRC) and chief of the VRC’s Cellular Immunology Section. 

The investigators analyzed the efficacy of CIS43LS two ways. Based on the time to first P. falciparum infection over the 24-week study period, the high dose (40 mg/kg) of CIS43LS was 88.2% effective at preventing infection and the lower dose (10 mg/kg) was 75% effective. An analysis of the proportion of participants infected with P. falciparum at any time over the 24-week study period found the high dose was 76.7% at preventing infection and the lower dose was 54.2% effective.

“These first field results demonstrating that a monoclonal antibody safely provides high-level protection against intense malaria transmission in healthy adults pave the way for further studies to determine if such an intervention can prevent malaria infection in infants, children, and pregnant women,” Dr. Seder said. “We hope monoclonal antibodies will transform malaria prevention in endemic regions.”