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BACKGROUND: Complement-fixing antibodies are important mediators of protection against Plasmodium falciparum malaria. However, complement-fixing antibodies remain uncharacterised for P. vivax malaria. Plasmodium vivax merozoite Surface Protein-3alpha (PvMSP3alpha) is a target of acquired immunity and a potential vaccine candidate. METHODS: Plasma from children and adults with P. vivax malaria in Sabah, Malaysia, were collected during acute infection, 7 and 28 days following drug treatment. Complement-fixing antibodies and IgM and IgG, targeting three distinctive regions of PvMSP3alpha were measured by ELISA. RESULTS: Seroprevalence of complement-fixing antibodies was highest against PvMSP3alpha Central region (77.6%). IgG1, IgG3, and IgM were significantly correlated with C1q-fixation and both purified IgG and IgM were capable of mediating C1q-fixation to PvMSP3alpha. Complement-fixing antibody levels were similar between age groups, but IgM was predominant in children and IgG3 more prevalent in adults. Functional antibodies increased following acute infection to 7 days after treatment, however rapidly waned by day 28. CONCLUSION: Our study demonstrates PvMSP3alpha antibodies acquired during P. vivax infection can mediate complement-fixation and shows the important influence of age in shaping these specific antibody responses. Further studies are warranted to understand the role of these functional antibodies in protective immunity against P. vivax malaria.