Malaria is one of the oldest and deadliest diseases, killing about half a million people each year. Researchers have long been focused on developing a vaccine for malaria and this week, in a momentous moment for modern medicine, the World Health Organization (W.H.O) has finally approved the first malaria vaccine.

The new immunization, made by GlaxoSmithKline, is specifically targeted towards protecting children and prompts the immune system to fight off Plasmodium falciparum, the deadliest of five malaria pathogens. This pathogen is also the most common in Africa, which is critical as nearly all malaria deaths occur in sub-Saharan Africa.

In clinical trials, the vaccine had a 50 percent efficacy rate against severe malaria. Unfortunately, this drops to nearly zero by four years after inoculation. According to Dr. Mary Hamel, who leads the W.H.O.’s malaria vaccine implementation program, the severity of malaria infection is “a reliable proximal indicator of mortality,” so a vaccine with 50 percent efficacy against severe infection will have a positive influence on mortality rates.

While 50 percent efficacy doesn’t sound very promising, a modeling study found that if the vaccine were rolled out in countries with the highest rates of malaria infection, 5.4 million cases and 23,000 deaths would be prevented in children younger than five each year. The protection against malaria can also be boosted by combining the vaccine with other measures such as preventive drugs given to children during high-transmission seasons and the distribution of bed nets, which cut malaria death in children under five by 20 percent.

Malaria is a particularly difficult disease to address because it can reinfect a person over and over again. In many parts of sub-Saharan Africa, children average six malaria episodes a year, also making them more vulnerable to other diseases. This new vaccine is groundbreaking as not only the first malaria vaccine to progress past clinical trials, but also as the first vaccine for any parasitic disease.

The vaccine, called Mosquirix, is given in three doses between 17 months and five years, with a fourth dose 18 months later. Clinical trials administered the vaccine to nearly 800,000 children in Kenya, Malawi, and Ghana with promising results. Researchers were optimistic about the ease of incorporating the vaccine into regular immunization schedules, a factor they anticipated might be challenging with four doses.

Moving forwards, the vaccine must be approved by Gavi, the global vaccine alliance, before it can be distributed to countries who request it.