*Purpose: Organ transplant recipients (OTR) have worse outcomes from COVID-19 and weaker antibody responses to vaccination than do immunocompetent individuals. Data on clinical outcomes among OTR with breakthrough COVID-19 are urgently needed, given decreased vaccine efficacy against the B.1.617.2 (Delta) variant. We compared crude case fatality rates (CFR) between fully vaccinated and unvaccinated kidney transplant recipients (KTR) with COVID-19.

*Methods: We identified KTR with COVID-19 at our institution between 3/1/20 and 11/17/21. Multi-organ transplant recipients, KTR who received additional (“booster”) doses, and those with partial or unknown vaccination status were excluded due to small numbers. KTR were considered fully vaccinated 2 weeks after receiving either the second dose of an mRNA vaccine series (Moderna, Pfizer-BioNTech) or one dose of the Janssen viral vector vaccine. Demographics, clinical characteristics, and in-hospital or hospice care mortality were extracted from electronic medical records.

*Results: Among 109 KTR with COVID-19, 19 were fully vaccinated at symptom onset. Vaccinated KTR with COVID-19 were older (median: 63.5 vs. 57.5 years, P<0.05) and waited longer to seek care after symptom onset (median: 6 vs. 3 days, P<0.05). Comorbidities and time from transplant were comparable between the two groups. CFR was higher among vaccinated KTR (26% vs. 10%, HR 0.34, 95%CI 0.11-1, P=0.05; Fig. 1), although the difference was not significant after adjustment for age (aHR 0.53, 95%CI 0.17-1.61, P>0.1). All fatal breakthrough infections occurred when the Delta variant accounted for >98% of COVID-19 cases in our HHS region.

*Conclusions: Vaccinated OTR remain at high risk for fatal COVID-19. Younger OTR are likely more immunoprotected than older OTR, which—combined with the emergence of the Delta variant and easing of restrictions—may have contributed to the observed shift toward older age among KTR with breakthrough COVID-19 and the high resultant CFR. Vaccinated OTR may delay seeking care for breakthrough symptoms due to a false sense of security. Our findings highlight the importance of pretransplant vaccination, and, among OTR, the need for ongoing preventive measures (masks, social distancing, vaccination of close contacts, post-vaccine education) and additional vaccine doses. OTR should be linked to care immediately after exposure or onset of symptoms consistent with COVID-19, given the availability of anti-spike monoclonal antibodies for prevention or treatment.

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