The world has gained a new weapon in the war on malaria, among the oldest known and deadliest of infectious diseases when the World Health Organisation (W.H.O) on Wednesday endorsed the first vaccine shown to help prevent the disease.

The vaccine, called Mosquirix, is not just a first for malaria — it is the first developed for any parasitic disease. Parasites are much more complex than viruses or bacteria, and the quest for a malaria vaccine has been underway for a hundred years.

Malaria kills about half a million people each year, nearly all of them in sub-Saharan Africa — including 260,000 children under 5. The new vaccine, made by GlaxoSmithKline, rouses a child’s immune system to thwart Plasmodium falciparum, the deadliest of five malaria pathogens and the most prevalent in Africa.

Dr. Tedros Adhanom Ghebreyesus, the World Health Organization’s director general, said the long-awaited vaccine was a breakthrough for science and could save tens of thousands of young lives each year.

Malaria is rare in the developed world. There are just 2,000 cases in the United States each year, mostly among travellers returning from countries in which the disease is endemic.

To have a malaria vaccine that is safe, moderately effective and ready for distribution is “a historic event,” said Dr. Pedro Alonso, director of the W.H.O.’s global malaria program.

“It’s a huge jump from the science perspective to have a first-generation vaccine against a human parasite.”

In clinical trials, the vaccine had an efficacy of about 50 percent against severe malaria in the first year, but the figure dropped close to zero by the fourth year. And the trials did not directly measure the vaccine’s impact on deaths, which has led some experts to question whether it is a worthwhile investment in countries with countless other intractable problems.

But severe malaria accounts for up to half of malaria deaths and is considered “a reliable proximal indicator of mortality,” said Dr. Mary Hamel, who leads the W.H.O.’s malaria vaccine implementation program. “I do expect we will see that impact.”

The malaria parasite, carried by mosquitoes, is a particularly insidious enemy, because it can strike the same person over and over. In many parts of sub-Saharan Africa, even those where most people sleep under insecticide-treated bed nets, children have on average six malaria episodes a year. ( The New York Times Company)