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Background Multiple myeloma (MM) patients have variable responses to mRNA vaccination to COVID-19. Little is known regarding their vaccine-induced antibody levels over time. Methods We monitored spike IgG antibody levels over 24 weeks among a subset of 18 MM patients who showed a full response after two mRNA vaccinations. MM patients had a more rapid decline in antibody levels as compared to 8 healthy controls, with power law half-lives of 72 days (versus 107 days) and exponential half-lives of 37 days (versus 51 days). Results The patients with longer SARS-CoV-2 antibody half-lives were more likely to have undetectable monoclonal protein than those with shorter half-lives, suggesting better disease control may correlate with longer duration of vaccine-induced antibodies. Regardless, by 16 weeks post-second dose of mRNA vaccination, the majority of patients had antibody levels below 250 binding arbitrary units per milliliter, which would be unlikely to contribute significantly to preventing COVID-19. Conclusions Thus, even MM patients who respond adequately to vaccination are likely to require more frequent booster doses than the general population.