Abstract Background We have reported that seroconversion rates after the second dose of mRNA‐based COVID‐19 vaccines for myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) were 100% and 95% respectively, with no significant difference from healthy controls (HCs).However, there are very limited data for the response to a third vaccine dose in those patients. Aims In this complementary study, we investigated the booster effect of a third mRNA‐based COVID‐19 vaccine dose in patients with myeloid malignancies. Materials & Methods A total 58 patients including 20 patients with MDS and 38 patients with AML were enrolled. Anti‐SARS‐CoV‐2S immunoassays were performed at 3, 6, and 9 months after the second vaccine dose. Results Seventy‐five percent of the MDS patients and 37% of the AML patients were receiving active treatment at the time of the third vaccination. Both the initial and third vaccine response in AML patients were comparable to those in HCs. In MDS patients, although the initial vaccine immunogenicity was inferior to that in HCs and AML patients, the third vaccine improved the response to a level not inferior to those in HCs and AML patients. Of note, the third vaccine resulted in a significant increase of antibodies in actively treated MDS patients who had shown a response inferior to that in untreated patients after two doses of vaccination. Discussion In patients with myeloid malignancies, the third vaccine dose showed a booster effect, and disease‐ and therapy‐related factors associated with the booster response have been identified. Conclusion The third dose of an mRNA‐based COVID‐19 vaccine showed a booster effect in patients with myeloid malignancies. Such a good booster response has not been reported in other haematological malignancies.
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