Explained: Malaria and the difficulty of developing a vaccine

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The World Health Organization Wednesday allowed “widespread use” the world’s first vaccine against malaria, a common mosquito-borne disease that kills more than four lakh each year. Developed by GlaxoSmithKline, the vaccine, known as RTS, S / AS01, has already been given to nearly 8 lakh children in Ghana, Kenya and Malawi in a pilot program since 2019.

The WHO approval paves the way for the use of this vaccine outside the pilot program, in all areas where malaria is known to be widespread. But the RTS.S / AS01 vaccine, known by its trade name Mosquirix, is only seen as the first step towards effective immunization of the world’s population against malaria. This vaccine is able to prevent severe cases of malaria in only 30% of cases, and the search for more effective vaccines is still ongoing.

The biggest killer

Malaria is known to be one of the deadliest diseases in human history, claiming millions of lives. Even now, the disease kills more than four lakh each year, according to WHO figures. This is still a huge improvement over twenty years ago, when nearly double the number of people died from the disease.

Malaria is most endemic in Africa, with Nigeria, Congo, Tanzania, Mozambique, Niger and Burkina Faso together accounting for more than half of annual deaths.

India is one of the countries hard hit by the disease. Malaria deaths have fallen sharply in recent years – officially only in the hundreds now – but infections continue to number in the millions.

Lack of vaccine

Despite decades of research, scientists have been unable to develop an effective vaccine against malaria, although more than 20 candidates have participated in clinical trials in recent years.

To date, the best prevention of malaria remains the use of mosquito nets. However, this does nothing to eradicate the disease.

Scientists cite various reasons for the failure to develop a malaria vaccine, the main one being the complexity of the life cycle of the parasite that causes malaria, part of which is spent in the human host.

“The difficulty in developing effective malaria vaccines stems in large part from the complexity of the life cycle of the malaria-causing parasites, which includes mosquitoes, human liver and human blood stages, and subsequent antigenic variations of the parasite. These parasites are also able to hide inside human cells to avoid being recognized by the immune system, creating new challenges, ”a group of Australian and Chinese researchers wrote in an open access journal of the year. last.

“Another challenge encountered in the development of a malaria vaccine is that the common human parasite causing malaria, P. falciparum, is not a rodent pathogen. The most common mouse models of malaria use the rodent-specific parasite species P. berghei, P. yoelii and P. chabaudi. These species generate unique pathologies and immune responses. Although they are still used to model various manifestations of human disease, the immune response patterns seen in these models are not fully transferable to humans, ”the researchers wrote.

Mosquirix itself is the result of over 30 years of research and development, yet has only modest effectiveness.

Other scientists, such as Navneet Arora, Lokhesh Anbalagan and Ashok Pannu, researchers from the Postgraduate Institute of Medical Education and Research (PGIMER) in Chandigarh cite other reasons such as lack of funding and interest in developing a malaria vaccine.

“Because malaria disproportionately affects LMICs (low and middle income countries) without a robust health infrastructure, vaccine manufacturers have little incentive to procure malaria vaccines and continue to target vaccines for markets. industrialized worlds, ”the three researchers wrote in an open access article. reviewed last year.

Other scientists have also mentioned that the search for a malaria vaccine has never received the same kind of attention as, say, HIV / AIDS.

A mother holds her baby in her arms receiving a new malaria vaccine as part of a trial at the Walter Reed Project Research Center in Kombewa, western Kenya. (AP Photo / Karel Prinsloo, file)

Malaria on the decline

However, significant progress has been made in recent years in reducing the impact of malaria. A few countries have also succeeded in eradicating malaria, mainly by spraying insecticides to kill mosquitoes and cleaning up areas where mosquitoes breed. Over the past twenty years, 11 countries have been declared by WHO to have become malaria-free, after no cases have been recorded in these countries for three consecutive years. These countries include the United Arab Emirates, Morocco, Sri Lanka, and Argentina.

In 2019, up to 27 countries reported less than 100 cases of malaria. Two decades ago, only six countries had less than this number.

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New vaccines

Several other vaccines are being tested. One of them also showed promise. In May of this year, a candidate malaria vaccine undergoing phase two trials reported 77% efficacy. This vaccine, R21 / Matrix M, is a modified version of Mosquirix and was developed by researchers at the University of Oxford. Principal researcher Adrian Hill, director of the Jenner Institute and professor of vaccinology at the University of Oxford, had said he believed the vaccine was the first to meet the WHO target of at least 75 % efficiency.

Dr VS Chauhan, former director of the Delhi-based International Center for Genetic Engineering and Biology, and well known for his efforts to develop a recombinant malaria vaccine, said that R21 / Matrix M showed great promise.

“This vaccine is certainly a great hope, but it has yet to go through phase three trials,” he said.

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