GlaxoSmithKline Plc’s experimental malaria vaccine becomes less effective over time, researchers in Kenya found, indicating that booster shots may be needed.

The efficacy of the shot, known as RTS,S, drops to zero by the fourth year after vaccination from 44 percent in the first year, according to a study published Wednesday in the New England Journal of Medicine. Even so, the researchers found that the medicine helped prevent 65 cases of malaria for every 100 children vaccinated.

The results suggest the need for a booster dose to sustain the vaccine’s protective effect, which Glaxo is exploring in a larger late-stage study. RTS,S is the most advanced vaccine in development among those targeting malaria, which kills 660,000 people a year worldwide, mostly children in sub-Saharan Africa, according to the World Health Organization.

“Generally, in other malaria studies there has been a trend to wane over time, so this is not surprising,” Johanna Daily, who studies malaria at Albert Einstein College of Medicine in New York and wasn’t involved in the research, said in a telephone interview. “The field has to reflect on this, whether there are other vaccines that will be more potent, that will have more prolonged efficacy.”

The researchers looked at 320 children in Kilifi, Kenya, in a followup to a previous study. The children, who were ages five months to 17 months at the time of vaccination, received three doses of the vaccine.

Different Settings

The study analyzes data from one trial site and doesn’t provide definitive answers about the vaccine’s duration of protection or how it works in different settings, Catherine Hartley, a Glaxo spokeswoman in London, said in an e-mailed statement. The company’s U.S. headquarters are in Research Triangle Park.

The larger, late-stage study involves 15,460 children “and should provide more meaningful insights into the vaccine candidate’s efficacy in different malaria parasite transmission settings, longer-term efficacy and the impact of a booster dose,” Hartley said. That analysis should be available by the end of 2014, she said.
The vaccine’s efficacy fell in the Kenyan study with increasing exposure to malaria, from 45 percent among children with below-average contact to the disease to 16 percent among those with above-average exposure.

“These are important findings that will help to inform which populations are likely to benefit most from the vaccine,” Ally Olotu, the study’s lead author and a doctoral student at the KEMRI-Wellcome Trust Research Program, said in a statement.

Fever, Chills

In November, results from the late-stage study were published, showing that the vaccine was less effective in reducing illness in infants than in older children.

There were 216 million cases of malaria globally in 2010, according to the WHO. The mosquito-borne parasite of the human liver and blood causes fever, chills and flu-like symptoms, or shock, anemia and organ damage if untreated. Malaria is prevented now with insecticide-treated bed nets. Drugs are prescribed after infection.

Glaxo has said it plans to file for regulatory approval in 2014 and bring the vaccine to market as early as 2015. The drugmaker hasn’t disclosed what it plans to charge for RTS,S, only that the eventual price will be the cost of making it plus 5 percent, which the company would reinvest in finding a second- generation malaria vaccine or preventive treatments for other neglected tropical diseases.