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Transmission-blocking vaccines push towards eradication

5 Nov 2009

Priya Shetty

Source: TropIKA

The first-ever phase 3 trial of a malaria vaccine has begun in seven countries across Africa, researchers announced this week at the MIM malaria conference in Nairobi. The trial of RTS,S, which showed 50% efficacy in phase 2 trials, is underway in Burkina Faso, Gabon, Ghana, Kenya, Malawi, Mozambique, and Tanzania. Investigators have already enrolled 5,000 of a total 16,000, Ashley Birkett, director of preclinical development at the PATH Malaria Vaccine Initiative (MVI) told TropIKA.

The trial forms a pivotal part of MVI’s vaccine efforts, which were reinvigorated in 2007 when the Bill and Melinda Gates Foundation called for a push towards eradication. The goals of elimination and eradication, rather than just control, has sparked new interest in transmission-blocking vaccines that stop the mosquito from spreading disease.

A transmission-blocking vaccine would work by stimulating antibodies to the stage of the parasite that replicates in the mosquito. When the mosquito bites a vaccinated person, it takes up both the parasite and the antibodies that kill them, thus stopping replication. MVI, which acts as a broker to drive vaccine development, is in talks with two companies to start trials on this type of vaccine.

Both RTS,S and transmission-blocking vaccines rely on the “Achilles heel of the life cycle of the parasite at two stages: when it goes from mosquito to human and then when it goes from human to mosquito,” says Birkett. RTS,S targets the first stage, and transmission-blocking vaccines hones in on the other.

For many of the delegates at the MIM conference, the holy grail would be to develop a combination vaccine that targets several stages of the parasite’s life cycle. “One way to build on RTS,S could be to improve the immune response to the target CSP antigen, or add other antigens whether they be pre-erythrocytic-stage, liver-stage, or blood-stage antigens.”

The success of GlaxoSmithKline Biological’s RTS,S vaccine has set a new benchmark for how effective a vaccine needs to be, says Birkett. But what everyone wants to know is when the world will have a usable malaria vaccine. Birkett estimates it will take another 4-5 years, but says that countries need to start getting ready for the vaccine now. This is a topic that exercises many malaria researchers – the fact that even well established-medicines like ACTs don’t get to the people who really need them because of price barriers or lack of transport and health infrastructure. MVI and GSK have taken this on board, says Birkett, creating a vaccine that when ready won’t require any more infrastructure than Africa already has.

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