A malaria vaccine developed by researchers at the University of Oxford’s Jenner Institute had up to 77% efficacy in a small clinical trial among children in Burkina Faso. The findings, posted as a preprint last week (April 20), represent the highest efficacy of any vaccine for the disease, although some researchers have cautioned that more data are needed before drawing firm conclusions about just how well it works.

The study results are “very positive news,” Pedro Alonso, the director of the World Health Organization (WHO) Global Malaria Programme, who was not involved in the work, tells Science. The trial only included 450 children, he adds. “We are still quite far away from having the type of information that would allow us to get very excited.” 

Malaria kills more than 400,000 people each year. Most of those deaths are in Africa, and most are among young children. Previous attempts to develop a vaccine have been hindered by the complexity of the malaria parasite—any of several species in the genus Plasmodium—which invades host cells and whose genome contains thousands of genes. 

It’s a “real technical challenge,” study coauthor Adrian Hill, a vaccinologist and director of the Jenner Institute, tells BBC News. “The vast majority of vaccines haven’t worked because it’s very difficult.”

The WHO has previously set a target of 75% efficacy for a malaria vaccine. Efficacy is measured as the reduction in disease incidence in vaccinated people compared to unvaccinated ones in a trial.

Before the current study results were posted, the most efficacious vaccine was GlaxoSmithKline’s Mosquirix, which showed around 56% efficacy in Phase 3 trials with young children after a year of immunization and is currently being deployed in several sub-Saharan countries. Follow-ups with immunized children showed that the vaccine prevented around 39% of malaria cases and 29% of severe cases in the four years following immunization.

The new vaccine is structurally similar to Mosquirix, consisting of hepatitis B surface proteins that self-assemble into virus-like particles and present part of a malaria protein. The Oxford vaccine is administered along with an adjuvant called Matrix-M, made by Novavax, to boost the immune response. 

Children in the current trial were split into three groups, two of which received the malaria vaccine and a high or low dose of the adjuvant, and one of which acted as a control and received a rabies vaccine instead. Participants received three doses, four weeks apart, followed by a booster dose about a year after the third jab.

Malaria vaccine efficacy among children who received a high dose of the adjuvant was 77% after a year of follow-up, while efficacy in children who got the lower dose of adjuvant was 71%.

“I am proud of Burkina Faso researchers who made a great contribution to reach this important milestone,” Alkassoum Maiga of the Ministry of Higher Education, Scientific Research and Innovation in Burkina Faso says in a statement. 

The study authors, along with researchers at Novavax and the Serum Institute of India Private Ltd, have started recruitment for a Phase 3 trial, which will include 4,800 children across four African countries, according to the statement. Maiga says that he hopes the trial “will confirm these exciting findings and that this vaccine could have a real impact on this disease affecting millions of children every year.”

Some researchers have called for more information about how the vaccine works. Rhoel Dinglasan, a malaria researcher at the University of Florida’s Emerging Pathogens Institute, tells Science that researchers would need more data—including genomic data from the parasite—to understand the vaccine’s efficacy against different strains, for example. “Where’s the biology?” he says.