Background/Objectives: This prospective study investigated the impact of high-dose chemotherapy and autologous stem cell transplantation (ASCT) on anti-COVID-19 antibody levels in previously vaccinated multiple myeloma (MM) patients with confirmed antibody response (AR). Methods: All patients underwent at least a two-dose regimen mRNA vaccination and later received a high-dose melphalan conditioning regimen and ASCT. Results: Fourteen MM patients with confirmed AR underwent a total of nineteen ASCT reinfusions; their median age was 55 (34–67). The study found a significant and progressive decrease in antibody levels after ASCT, from 311 BAU/mL at baseline to 276 BAU/mL and 188 BAU/mL after one and three months, respectively, with a median anti-COVID-19 antibody level reduction of 39% (range 16–66%) that was statistically significant (p = 0.014) using the Friedman test. However, the third “booster” vaccination post-ASCT improved the humoral response at six months in nine patients (50% response rate) and corrected, at least in part, the negative impact of high-dose chemotherapy (p = 0.597). Despite the antibody decline, three patients who contracted COVID-19 after ASCT experienced mild, outpatient-managed infections, suggesting sufficient immune response. Furthermore, booster doses increased the proportion of high-responders (AR > 500 BAU/mL) post-ASCT from 22% to 55% (5/9 patients) at three and six months, respectively. Conclusions: The study concludes that ASCT negatively affects the humoral response, but booster vaccination can improve it, and residual antibodies may prevent severe COVID-19 in these vulnerable patients.