VOA英语2010年-Malaria: New Partnership Aims to Stop Parasi

A new partnership 1 is being launched to develop a malaria 2 vaccine 3 that would prevent parasites 5 from entering the liver. Most of the symptoms of the disease develop after parasites have had time to replicate 6 there.

A new partnership is being launched to develop a malaria vaccine that would prevent parasites from entering the liver. Most of the symptoms of the disease develop after parasites have had time to replicate there.

The World Health Organization estimates that in 2008, there were 247 million cases of malaria worldwide, with one million deaths. It says in Africa, a child dies every 45 seconds from the disease.

The best treatment now available is artemisinin-based combination therapy. But an effective vaccine would be much better. That's the goal of the partnership involving the international, non-profit organization PATH, Merck Pharmaceuticals 7 and New York University.

Finding a weakness

Scientists think they've discovered the Achilles heel of the malaria parasite 4. After a bite by an infected mosquito, the parasite travels through the bloodstream and arrives at the liver. That's where scientists believe the best line of defense 8 can be made.

Dr. Ashley Birkett is PATH's Malaria Vaccine Initiative director of pre and early clinical development.

"The parasite has a natural affinity 9 for liver cells. Liver cells are the cells that the parasite can actually make copies of itself in once it enters the human body. So, it's goal once it enters the human bloodstream after an infectious bite is to find liver cells. And then it enters those liver cells through a set of specific interactions that we don't fully 10 understand. But there is a critical event in that entry process that we think we can target through a vaccine approach," he says.

He says if the parasite cannot enter the liver, it cannot replicate and cause disease. But targeting a parasite is more difficult than targeting viruses or bacteria.

"This is a parasite," he says, "It's a very complex organism that has over 5,000 different gene 11 products that it makes. It's a lot more complicated than a typical virus. I think also there have been limited resources that have been applied 12 to malaria vaccine development over the years. But we are having significant success now. And through a partnership with GlaxoSmithKline, PATH MVI is engaged in a very large phase-three clinical study, which is a last stage of clinical testing before a vaccine can actually be licensed 13 and start saving lives."

Birkett says the vaccine has been evaluated in 16,000 young children in seven sub-Saharan African countries. It has an efficacy rate of 50 percent.

"So, given that we're looking in the order of 900,000 deaths from malaria in young children primarily per year in sub-Saharan Africa, a vaccine with 50 percent efficacy is actually a very significant achievement. But we want to do better," says Birkett.

The vaccine being tested depends on the body producing very large numbers of antibodies against the malaria parasite. That's makes it more difficult to have a higher efficacy rate. The hope of a new vaccine is to specifically target a different area of the parasite with fewer anti-bodies.

Malaria taking a toll 14

Dr. Birkett says this would relieve African countries of some heavy burdens.

"We're looking at countries in sub-Saharan Africa, which already have significant burdens to deal with in terms of economic impact of these global infectious diseases. And malaria has a huge economic impact. So a vaccine that can be highly protective and prevent these clinical diseases and up to 900,000 deaths per year is going to have a significant health and economic benefit in these countries," he says.

Dr. Birkett says the cost of such a vaccine would be kept as low as possible. Clinical testing on the proposed vaccine is still three or four years away. Malaria immunization would be included as part of a child's routine vaccinations 15.