By Denise Roland 

LONDON-- GlaxoSmithKline PLC's malaria vaccine, the world's most advanced, has cleared its final scientific hurdle, but it could still take up to a year before immunization programs begin in sub-Saharan Africa.

The vaccine, which is aimed at young children, received a positive scientific opinion from the European Medicines Agency's Committee for Medicinal Products for Human Use on Thursday. But the eventual rollout of the vaccine, called RTS,S, depends on a series of decisions by stakeholders including the World Health Organization, individual governments in sub-Saharan Africa, and GAVI, the Vaccines Alliance, which is expected to subsidize the immunization programs.

The WHO "may introduce certain restrictions or specifications like [to] start in highly endemic areas, or...give it in combination with other vaccines given in the same age bracket," said Moncef Slaoui, Glaxo's chairman of global vaccines. WHO officials have also said the agency will consider how to ensure the vaccine is used alongside other strategies such as the provision of mosquito nets and antimalarial drugs.

Once the WHO has made a policy decision, which could take several months, GAVI will consider its strategy for the vaccine. While GAVI has not yet explicitly committed to funding the jab, it is widely expected to do so.

Meanwhile, individual governments across sub-Saharan Africa will determine their own immunization strategies.

The Glaxo vaccine, which is supported by the PATH Malaria Vaccine Initiative and the Bill and Melinda Gates Foundation, is the most advanced anywhere in the world by several years and the first against any parasitic disease.

However, it is no magic bullet. In clinical trials, it was shown to prevent around half of all cases of malaria in children given their first shot aged 5 to 17 months, and a quarter of cases in those aged 6 to 12 weeks when first inoculated. That is a much lower level of protection than the 80% disease prevention from most routine immunizations. Moreover, it loses effectiveness over time, with the level of protection in toddlers dropping to just 36% after four years, and is less potent against severe malaria, the life-threatening form of the disease.

Nonetheless, scientists have said the vaccine offers a clear benefit to children in sub-Saharan Africa because even this relatively low level of protection could prevent hundreds of thousands of cases of malaria every year. In 2013, around 437,000 African children died before their fifth birthday due to malaria, most of which were in the sub-Saharan region, according to the WHO.

The vaccine could also prove more effective when used in real-world conditions, according to Dr. Slaoui. He said the clinical trials will have likely played down the efficacy of the vaccine since the trial sites were optimized for the prevention of malaria through, for example, the provision of treated mosquito nets that were frequently checked for holes.

"The reality is that the bed net is only effective if you have used it," Dr. Slaoui said. "With vaccines, once immunized, your protection is with you day and night. My expectation is the effectiveness of this vaccine in real life conditions is going to be substantially higher."

To determine the vaccine's real-world effectiveness, Glaxo has recommended to the WHO that it be introduced in a "scientifically driven way." That would mean first deploying the vaccine in areas with health-care facilities that could monitor children after immunization. "We would not support widespread immediate use. The data will inform how best to use this vaccine," Dr. Slaoui said.

The launch of immunization programs using Glaxo's vaccine, which Dr. Slaoui hopes will happen by mid-2016, would mark the culmination of nearly three decades of development efforts, with research partners including the Walter Reed Army Institute of Research in Silver Spring, Md. RTS,S has taken around twice as long as a typical vaccine because the disease is caused by a complex parasite.

Glaxo has already manufactured tens of millions of doses of the vaccine in anticipation of its launch and is continuously producing more at its facility in Belgium, according to Dr. Slaoui. "We are preparing to be able to provide the vaccine when it's needed," he said.

Glaxo is the only big pharmaceutical company with a malaria vaccine, but a number of new approaches are being pursued by nonprofits, government programs and universities, such as the National Institute of Allergy and Infectious Diseases in the U.S. and the University of Oxford. Sanaria Inc., a small biotech company based in Rockville, Md., also is developing a malaria vaccine.

Glaxo has said it plans to sell the vaccine at a 5% premium to the manufacturing cost and use the profits to fund further research into vaccines for malaria and other neglected tropical diseases.

Write to Denise Roland at Denise.Roland@wsj.com

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