AbstractBackgroundMultiple myeloma (MM) patients have variable responses to mRNA vaccination to COVID‐19. Little is known regarding their vaccine‐induced antibody levels over time.MethodsWe monitored spike IgG antibody levels over 24 weeks among a subset of 18 MM patients who showed a full response after two mRNA vaccinations.ResultsMM patients had a more rapid decline in antibody levels as compared to eight healthy controls, with power law half‐lives of 72 days (vs. 107 days) and exponential half‐lives of 37 days (vs. 51 days). 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 to preventing COVID‐19.ConclusionsThus, even MM patients who respond adequately to vaccination are likely to require more frequent booster doses than the general population.