New Delhi, Sept. 7: In the decade since Virender Chauhan plunged into the race to make a vaccine against malaria, the scientific challenge that the effort has posed appears to have turned tougher than anyone had imagined.
When asked to deliver a talk on prospects for a malaria vaccine, Chauhan, the director of the International Centre for Genetic Engineering and Biotechnology (ICGEB), New Delhi, typically starts by listing reasons why the effort has been so difficult.
“Over thousands of years, the malaria parasite has evolved smart strategies to evade the human immune system,” says Chauhan. The parasite changes its form in the human body and virtually plays games with the human immune system. Scientists do not have a suitable animal model. And attempts in the past to protect people from malaria with vaccines have failed.
“But we’re not at the doorstep of defeat either,” asserts Chauhan, who heads a research team at the ICGEB that has developed an experimental vaccine against Plasmodium vivax, the parasite that causes the most widespread form of malaria.
The vaccine is among a number of new “candidate” vaccines that, scientists expect, will enter human clinical trials within the next five years.
Ten projects aimed at producing vaccines are currently supported by the Malaria Vaccine Initiative (MVI), an effort funded by the Bill and Melinda Gates Foundation. Research teams and drug companies from Australia, the US, the UK and Africa are in the race for the vaccine.
Nine of the MVI-supported projects are aimed at vaccines for Plasmodium falciparum, the parasite that causes the most lethal form of malaria that can affect the brain. But Plasmodium vivax accounts for 60 per cent of malaria cases in India.
The ICGEB vaccine is intended to prevent Plasmodium vivax from invading red blood cells, the first step in the disease cycle after the parasite enters the body through mosquito bites.
Over the past two decades, researchers elsewhere in the world have developed, tested and abandoned half a dozen malaria vaccines. Some of the vaccines did produce an immune response ' when injected into humans, they produced antibodies against the parasite, but the antibodies did not protect the immunised people from disease when subsequently exposed to malaria.
“Recent studies have shown that the malaria parasite can suppress certain elements of the human immune system,” says Shobhona Sharma, a biologist at the Tata Institute of Fundamental Research in Mumbai. “It also dramatically transforms itself in the body.”
Malaria parasites enter the human body through mosquito bites in the form of tiny worm-shaped creatures called sporozoites. They invade the liver where they multiply for about a week and emerge as tear-shaped merozoites.
Sharma says the interactions between the parasite and the immune system are being unravelled. “I don’t think we’re going to have an effective vaccine for another 15 or 20 years,” she says.
Yet, scientists say there are reasons for optimism. People who have lived for many years in high-risk areas for malaria develop a temporary immunity. Such people harbour high loads of Plasmodium falciparum in their blood or liver but don’t fall sick.
But such “natural immunity” is short-lived. When such naturally-immunised people move out of the high-risk zones, they lose their immunity within a year and can become sick when infected with a parasite.
Natural immunity shows that there are mechanisms through which the human body can protect itself from the parasite. “When nature itself provides short-lasting immunity, we’ve got to do better than nature,” says Chauhan.
Another reason for hope is that blood from infected patients has been shown to contain antibodies that can prevent the parasite from infecting blood cells.
Researchers believe that pursuing different vaccine design strategies might lead to something that works. “The initial goal is a vaccine to artificially build up an immune response to protect children from severe disease and death,” says Chauhan.
Eventually, says Chauhan, research teams now working on independent projects might have to work together and develop a vaccine that combines different strategies.