*Purpose: Some solid-organ transplant recipients (SOTRs) with low or negative antibody levels after a 2-dose mRNA vaccine series against SARS-CoV-2 experience boosting after a third dose (D3), but long-term antibody durability after D3 is unknown. We describe six-month SARS-CoV-2 antibody kinetics and durability in 31 SOTRs who received D3.

*Methods: 31 SOTRs without prior COVID-19 were identified within our national observational study. Serologic testing was performed a median of 30 (IQR 27-40) days after D3 and repeated at a median of 166 (148-184) days after D3. Semiquantitative anti-spike serologic testing using the Roche Elecsys anti-S enzyme immunoassay (EIA) or EUROIMMUN anti-S1 EIA was performed.

*Results: Over 6 months of follow-up, antibody levels increased in 16/27(59%), remained stable in 6/27(22%) (one negative, the others above the assay limit), and decreased in 5/27(19%). One-month post-D3, 24/31(77%) were seropositive and 7/31(23%) were seronegative. Six-months post-D3, 29/31(94%) were seropositive and 2/31(6%) remained seronegative. Both nonresponders received the BNT-162b2 primary series; one received Ad.26.CoV2.S and the other mRNA-1273 for D3. This difference in seroconversion after D3 was not statistically significant (Fisher exact = 0.49, between primary series). There were no reported cases of COVID-19 during the study period.

*Conclusions: We observed a very high rate of seroconversion after D3 in SOTRs, with marked heterogeneity in timing and strength of response depending on baseline antibody level and vaccine platform received. These results are encouraging evidence for the durable immunogenicity of additional vaccine doses in most SOTRs, and demonstrate the need for dedicated analysis of heterologous boosting strategies.

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