Kenya: Hope in the New Year As Malaria Vaccines Lined Up for Clinical Trials

31 December 2019

As this year comes to an end, we remain hopeful that malaria, an infectious disease that kills 1,200 children under five years every day, will be prevented using a vaccine.

Kenya, Ghana and Malawi rolled out a largescale trial of RTS, S -- a vaccine manufactured by GlaxoSmithKline that offers partial protection against malaria.

A suboptimal vaccine with clinical trial phase III showing 30 per cent efficacy, RTS, S is a big leap.

Another vaccine, R21, which is similar to RTS, S in terms of the antibodies it elicits, has also been unveiled.

R21 has a large portion of a plasmodium surface protein called circumsporozoite; the RTS, S has a smaller fragment of the protein, which is cited as a major limitation in eliciting strong antibody response.

IMMUNE RESPONSE

Circumsporozoite is the antigen in the sporozoite stage of malaria parasite that is injected into the human blood stream by mosquito bite.

The antigen triggers production of antibodies by the immune system, the antibodies then neutralise the sporozoites and enhance their removal from the body.

The main challenge with sporozoite-based vaccines is the window of time within which an immune response is supposed to be mounted.

The sporozoite lingers in blood stream for about 30 minutes before entering the liver, where the parasite changes into merozoites whose surface antigens cannot react with the sporozoite-triggered antibodies.

However, immunologists are working on the use of weakened but live malaria parasite vaccines.

In 2020, clinical trials of a live but attenuated malaria vaccine will be rolled out in Tanzania.

Laboratory studies show this to be more effective; the challenge is malaria is caused by five different plasmodium species, requiring different live attenuated plasmodium vaccines.

But this could be overcome by a mosaic vaccine that has antigens from the five species of plasmodium.

HUGE CHALLENGE

In yet another trial, PfSPZ, a vaccine based on irradiated plasmodium, will be rolled out in February in Biko Island, where malaria is endemic.

Lab tests have confirmed the vaccine to be 100 per cent effective, but again, there is the challenge of different malaria parasite species.

It may protect against plasmodium falciparum but not plasmodium vivax, for example.

Using a technique known as CRISPR-cas9, immunologists are editing the plasmodium genes to remove those that encode proteins that make merozoites to leave liver and kill red blood cells. This will make the parasites to remain in the liver longer.

Immunologists have identified the protein used by merozoites to harpoon red blood cells.

HOPE

RH5 is highly conserved in all the five species of plasmodium. Antibodies against RH5 can prevent the attack of red blood cells by merozoites, dampening clinical symptoms of malaria, like anaemia and fever.

A vaccine targeting RH5 is coming soon. There is hope of an affordable, safe, easy-to-administer and effective malaria vaccine soon.

Dr P.M. Mutua, immunologist, Makueni

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