Experimental malaria vaccine shows huge promise

Debora MacKenzie | 18 Oct 2006
New Scientist

A review of experimental malaria vaccines has found that one shows such promise, the results "justify speedy progress towards licensing for routine use". That would make it the first-ever commercial vaccine against one of the world's deadliest diseases - but it may need to be used along with other kinds of malaria vaccine to really stop the killer.

Malaria is a parasite carried by mosquitoes, and is thought to have killed more humans throughout history than any other disease. It now kills at least a million people a year, especially children in Africa, and increasingly resists anti-malarial drugs.

Efforts to develop a vaccine have been frustrated because the parasite changes its form several times during an infection. These changes alter the surface chemicals that vaccines normally exploit to prime the immune system to recognise a pathogen. Scientists are divided over which stage to vaccinate against.

Vaccinating against the first stage, the sporozoite, which enters the blood from the mosquito, might prevent infection entirely. But the immune system would have a limited time to work before the parasite changes into the next stage, the merozoite, which enters red blood cells. Vaccinating against merozoites would not prevent infection, but might limit disease.

The Cochrane Collaboration, an organisation that reviews clinical trials, has looked at recent experiments with four leading candidates for a sporozoite vaccine. Three, it said, show no clear effect. One, however, showed effectiveness.

Best protection yet

It consists of a large chunk of a surface protein from sporozoites which the scientists attached to a surface protein from the hepatitis B virus. The virus part stimulates the immune system, while the pathogen part primes the immune system to attack the sporozoite.

The vaccine protected 41% of volunteers in the US who were exposed to malarial mosquitoes in the lab. When it was tried on children in Mozambique, it cut malaria cases by 26% for 18 months after vaccination - and cut the severity of the illness by more than half. In Gambia, men given two shots, one year apart, suffered 63% fewer cases.

That is poor protection compared with the near-complete protection associated with polio or measles vaccines, but it is the best so far against malaria, the reviewers say, and means the company that owns it, Smith Kline Beecham, should get it into commercial use soon.

Protection might be better if the sporozoite vaccine can be combined with one against merozoites, thus hitting the parasite twice. In a separate review in the same issue of the Cochrane journal, the researchers report that the only merozoite vaccine so far tested for effectiveness did not reduce rates of illness in Australian volunteers, or in children in Papua-New Guinea. But it did cut the number of parasites in their blood.

Journal reference: The Cochrane Database of Systematic Reviews (2006, issue 4)