In a new modeling study published in Nature Communications, One Health Trust researchers and collaborators estimated the impact of a malaria vaccine in 42 African countries over a ten-year period in several scenarios with varying vaccine efficacy (VE) and levels of antimalarial drug resistance. The burden averted in Scenario 2, in which VE was higher right after vaccination at 80 percent and dropped by 20 percent each year, was greater than in Scenario 1, whereby the VE was constant at 40 percent for four years and dropped to 0 percent in year five. The second scenario approximates the VE values calculated in Phase II and III trials of the WHO-recommended RTS,S/AS01 and R21/Matrix-M vaccines, respectively, and resulted in 313.9 (uncertainty index: 249.8-406.6) malaria cases per 1000, 0.9 (UI 0.6-1.3) drug-resistant cases per 1000, and 0.9 (UI 0.6-1.2) deaths per 1000 averted by the vaccine. However, Scenario 3, in which VE remained constant at 40 percent for the length of the study, averted the most malaria cases (384.7 [UI 311.7-496.47]) per 1000, drug-resistant cases (1.0 [UI 0.7-1.6]) per 1000, and deaths (1.1 [UI 0.5-1.5]) per 1000 out of all tested scenarios.

A “worst-case” scenario was modeled with the same VE trend as Scenario 1 but with resistance to artemisinin combination therapies (ACTs) — measured by delayed parasite clearance rates — increasing on a linear scale to 80 percent by 2030. Modeling results of this scenario revealed more resistant cases (10.4 [UI 7.3-15.8]) per 1000 averted than in Scenario 1 (0.64 [0.4-1.0]) despite having the same VE trend.

Geographical differences and variations among human hosts cause significant variations in VE within and between countries. Understanding how different VE trends impact the burden of malaria on communities can help predict the results of malaria vaccine roll-outs. Development and distribution of an effective malaria vaccine must be fast-tracked in order to ameliorate the burden of ACT resistance on malaria-endemic regions around the world.

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