The latest clinical trial of the world’s leading malaria vaccine candidate produced disappointing resultson Friday. The infants it was given to had only about a third fewer infections than a control group.

But researchers said they wanted to press on, assuming they keep getting financial support, because the number of children who die of malaria is so great that even an inefficient vaccine can save thousands of lives.

Three shots of the vaccine, known as RTS, S or Mosquirix and produced by GlaxoSmithKline, gave babies fewer than 12 weeks old 31 percent protection against detectable malaria and 37 percent protection against severe malaria, according to an announcement by the company at a vaccines conference in Cape Town.

Last year, in a trial in children up to 17 months old, the same vaccine gave 55 percent protection against detectable malaria and 47 percent against severe malaria.

The new trial “is less than we’d hoped for,” Moncef Slaoui, chairman of research and development at Glaxo, said in a telephone interview. “But if a million babies were vaccinated, we would prevent 260,000 cases of malaria a year. This is a disease that kills 655,000 babies a year — 31 percent of that is a very large number.”

The company, which has already spent more than $300 million on the vaccine, wants to keep forging ahead, Mr. Slaoui said, “but it is not just our decision.”

It also depends on the PATH Malaria Vaccine Initiative, which has put more than $200 million of its Bill and Melinda Gates Foundation financing into the vaccine, and on the World Health Organization, which has helped talk seven African countries into allowing the vaccine to be tested on their children.

The Gates Foundation declined to say how much money it was ultimately prepared to spend on an imperfect vaccine; this set of trials is set to go into 2014.

“The efficacy came back lower than we had hoped, but developing a vaccine against a parasite is a very hard thing to do,” Bill Gates said in a prepared statement. “The trial is continuing, and we look forward to getting more data to help determine whether and how to deploy this vaccine.”

All the families in the trial were given insecticide-treated mosquito nets and encouraged to use them; 86 percent did, so the vaccine’s results were achieved on top of other anti-malaria measures.

RTS, S contains a protein found on the parasite’s surface that provokes an immune reaction. It was first identified decades ago by two New York University scientists, Ruth and Victor Nussenzweig. The vaccine was developed by Glaxo in Belgium and initially tested on American volunteers by the Walter Reed Army Institute of Research.

When the Gates Foundation began focusing on global health in the early part of this century, it was one of the first projects the foundation adopted. Different ways to make the vaccine more effective, including adding different boosters and giving more shots, are being experimented with. Other vaccines using different ways to provoke an immune reaction exist, but none are as far along in clinical trials.

Like an H.I.V. vaccine, one against malaria has proved an elusive goal. The parasite morphs several times, exhibiting different surface proteins as it goes from mosquito saliva into blood and then into and out of the liver. Also, even the best natural “vaccine” — catching the disease itself — is not very effective. While one bout of measles immunizes a child for life, it usually takes several bouts of malaria to confer even partial immunity. Pregnancy can cause women to stop being immune, and immunity can fade out if someone moves away from a malarial area — presumably because they no longer get “boosters” from repeated mosquito bites.

A version of this article appeared in print on November 10, 2012, on page A5 of the New York edition with the headline: Malaria Vaccine Candidate Gives Disappointing Results.



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