Malaria, a parasitic disease transmitted by infected female Anopheles mosquitoes, poses a global health challenge. In 2022, there were an estimated 249 million malaria cases worldwide, resulting in approximately 608,000 deaths, with young children in sub-Saharan Africa being the most affected demographic. Vaccines represent a significant advancement in preventing this widespread illness.
Current Malaria Vaccines
The global fight against malaria has seen the introduction of two significant vaccines recommended by the World Health Organization (WHO). The first, RTS,S/AS01, also known as Mosquirix, received a WHO recommendation in October 2021. This vaccine is specifically designed to prevent malaria caused by the Plasmodium falciparum parasite, which is the deadliest form of malaria and most prevalent in Africa. Mosquirix targets young children from 5 months of age in areas with moderate to high malaria transmission.
Following Mosquirix, the R21/Matrix-M vaccine became the second malaria vaccine to receive a WHO recommendation in October 2023. Like RTS,S/AS01, R21/Matrix-M also targets Plasmodium falciparum and is intended for use in children. The availability of both vaccines is expected to address the demand for malaria prevention tools, especially in African regions where the disease burden is highest.
How Long Protection Lasts
The duration of protection offered by malaria vaccines varies, and efficacy can wane over time for both RTS,S/AS01 and R21/Matrix-M. For the RTS,S/AS01 vaccine, a four-dose regimen is recommended: three initial doses given monthly, and a fourth dose around 15-18 months after the third. Studies have shown that for children aged 5-17 months who received four doses, vaccine efficacy against clinical malaria was around 36% over four years of follow-up.
The R21/Matrix-M vaccine has demonstrated high efficacy when administered seasonally. A three-dose series given just before the high transmission season has shown to reduce symptomatic malaria cases by 75% over 12 months in areas with highly seasonal transmission. For age-based schedules in areas with perennial transmission, vaccine efficacy is around 66% over 12 months after the initial three doses. A fourth dose given one year after the third has been shown to maintain this high efficacy.
Importance of Booster Doses
Booster doses extend and maintain the protection from malaria vaccines. Immunity to malaria can decline over time after the initial vaccination series, a phenomenon known as waning immunity. Booster shots work by re-stimulating the immune system, prompting it to produce more antibodies and memory cells, thereby reinforcing the body’s defense against the parasite.
For RTS,S/AS01, a fourth booster dose given approximately 15-18 months after the third dose is recommended to prolong protection. Similarly, for R21/Matrix-M, a booster dose administered 12 months after the primary three-dose series has been shown to maintain high efficacy. This strategy is valuable in regions with highly seasonal malaria transmission, where a booster can be timed to provide protection just before the peak malaria season.
Maintaining Protection After Vaccination
Vaccination is an important tool in malaria prevention, but it is part of a broader strategy. Even after vaccination, other prevention measures remain important to enhance protection. This multi-pronged approach helps to address the complex nature of malaria transmission.
Strategies include the use of insecticide-treated bed nets (ITNs), which provide a physical and chemical barrier against mosquito bites, especially during sleeping hours. Indoor residual spraying (IRS), where insecticides are applied to the interior walls of homes, also helps to kill mosquitoes that rest indoors. Prompt diagnosis and treatment of malaria cases are important to prevent severe illness and reduce the spread of the parasite within communities.