Will There Ever Be a Malaria Vaccine?

Malaria is a life-threatening disease transmitted to humans by Anopheles mosquitoes, and caused by four species of parasites: Plasmodium vivax (the most widely distributed), Plasmodium ovale, Plasmodium malariae and Plasmodium falciparum (responsible for most malaria-related deaths).

After an infected mosquito bite, malaria parasites spread through the bloodstream and infect the liver. They then multiply in red blood cells and cause damage to the nervous system, liver, and kidney.

Worldwide, 300 million people become severely ill with malaria each year and between 700,000 and 2.7 million of them die-75% of which are African children. Malaria kills an African child every 30 seconds. Also, pregnant women are specially susceptibility to Plasmodium falciparum malaria, which increases the risk of prematurity, abortion, and stillbirth.

Although malaria has been eradicated in the United States since the early 1950s, 1,337 cases of malaria and 8 deaths were reported in 2002. Most of these cases occurred among US travelers who were infected in malaria-endemic countries. For instance, a review by the Centers for Disease Control and Prevention (CDC) of data for malaria-related deaths from 1963 through 2001, found that of the 185 deaths for that period, 123 (66.5%) occurred among US travelers.

The risk of contracting malaria is highest for travelers to sub-Saharan Africa, Papua New Guinea, the Solomon Islands and Vanuatu, while intermediate in Haiti and the Indian subcontinent and low in most of Southeast Asia and Latin America.

Various strategies have been taken to prevent disease and death from malaria:

Mosquito control-either by using pesticides or cleaning up stagnant waters where mosquitoes breed.

Human exposure prevention-by avoiding mosquito bites with the use of window screens and bed nets, for example.
Disease prevention-by using antimalarial drugs such as chloroquine before exposure. However, this strategy is only recommended for travelers and not for people living in endemic areas.

However, none of these strategies has proved completely effective since mosquitoes develop resistance to pesticides and the parasites to antimalarial drugs. For that reason, efforts had focused on the development of a vaccine against malaria.

Malaria vaccine

There is currently no licensed vaccine for malaria, although several candidate vaccines are undergoing clinical trials.

Challenges for creating and effective malaria vaccine include:

The parasite's complexity-it has more than 6,000 genes, much more than common viral infections.

The parasite's ability to change through its life cycle both in the human and in the mosquito.

The parasite's ability to hide from the immune system.

Malaria vaccine candidates have targeted different stages of the parasite's life cycle to prevent or reduce disease or to block transmission. For example, some vaccine candidates seek to prevent infection of liver cells (pre-erythrocytic vaccines); others seek to prevent multiplication of the parasite in red blood cells (blood-stage vaccines); and others seek to neutralize the parasite in mosquitoes after they have bit an infected-and vaccinated-person (transmission-blocking vaccines).

One of the most advanced malaria vaccine candidates is the RTS,S/AS02A-a pre-erythrocytic vaccine. In a large clinical trial in Mozambique between 2003 and 2004, the RTS,S/AS02A vaccine protected a significant percentage of children 1 to 4 years of age against uncomplicated malaria, infection, and even severe forms of the disease for at least six months.