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And there's new information on a vaccine against one of the world's biggest health problems, malaria. It's the only malaria vaccine in large-scale human trials. The effect of the vaccine persists for as long as a year and a half and for some young children, it cut the number of cases by half - which is progress, though most childhood vaccines provide at least 90 percent protection. Still, as NPR's Richard Knox reports, experts say they are optimistic about the results.

RICHARD KNOX, BYLINE: It's taken nearly 30 years and roughly half a billion dollars to get to this point - solid results from a study of more than 15,000 children across Africa with a vaccine called RTSS. The new data, presented today at a conference in South Africa, show the vaccine is what researchers consider modestly effective, although immunity decreases slightly over time. But the vaccine's developers emphasize that it still prevents a lot of cases. Dr. David Kaslow is with the PATH Malaria Vaccine Initiative, a study sponsor.

DR. DAVID KASLOW: Nine hundred and forty-one cases prevented on average for, you know, every thousand young children vaccinated with RTSS, suggests that it has potential for impact; you know, pretty big public health impact. Those are some pretty big numbers of cases averted.

KNOX: Now, 940 cases of malaria prevented out of every thousand kids vaccinated sounds like the vaccine prevents more than nine out of 10 cases. But actually, it's more like one out of two because many African kids get malaria more than once a year. The new data add to results released last year, which gave the first indication the vaccine has limited effectiveness. But Dr. Moncef Slaoui of GlaxoSmithKline, the main sponsor, says even a vaccine of modest effectiveness can have a big impact because malaria is such a huge problem in Africa.

DR. MONCEF SLAOUI: Malaria in sub-Saharan Africa kills around 600,000 children every year, which is an enormous number.

KNOX: The project couldn't demonstrate the vaccine's impact on malaria deaths. That's because children in the study all got the very best treatment, so very few died of the disease. But those who got the vaccine were 36 percent less likely to get severe malaria.

SLAOUI: This vaccine, I believe, over time, will demonstrate in real life, in real use in the field, a very significant efficacy against death resulting from malaria infection. We're talking about very large numbers prevented.

KNOX: But everybody in the field acknowledges that it's not going to be easy to decide how to use the RTSS vaccine. The new data show that 18 months after vaccination, it's 46 percent effective in children immunized between 5 and 17 months of age. For infants, it was only 27 percent effective. Work is underway to see if a booster shot can raise those rates.

Dr. Johanna Daily, of Einstein Medical College, says the results are disappointing.

DR. JOHNANNA DAILY: There's no question everybody would want a higher efficacy on this vaccine. Standard vaccines have over 80 - sometimes 90 - percent efficacy, to protect from other infections. But malaria has always been such a challenge. And we may not ever get those levels.

KNOX: Even when people get infected with malaria - the most effective form of immunization - protection against future infection is far from complete.

DAILY: So it's hard to maybe improve upon nature. So perhaps hoping for a more efficacious vaccine is unlikely and therefore, this would be the thing to go with.

KNOX: Something like 20 other malaria vaccines are in the pipeline. But it's going to be awhile before anyone knows if they'll be more effective than RTSS. GlaxoSmithKline says it will ask European drug regulators next year to approve the vaccine. If they do, the World Health Organization will consider - probably in 2015 - whether to recommend its use.

Daily says malaria vaccine will have to compete with other measures, such as insecticide-treated bed nets to keep malaria-infected mosquitoes from sleeping children.

DAILY: I think it's going to be a very tough question. Each country is going to have a pot of money, and they're going to have to decide how to spend it. Should it go to vaccine? Should it go to bed nets? Should it go to diagnosis? Should it go to community health workers? Each minister of health can decide is this something they want to put their money towards.

KNOX: Glaxo has pledged to keep the cost of the vaccine low. It won't be pennies per dose, the company says; more likely, a few dollars.

Richard Knox, NPR News.

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