Rationale & Objective: SARS-CoV-2 infections are likely underdiagnosed, but the degree of underdiagnosis among maintenance dialysis patients is unknown. Durability of the immune response after third vaccine doses in this population also remains uncertain. This study tracked antibody levels to 1) assess the rate of undiagnosed infections and 2) characterize seroresponse durability after third doses. Study Design: Retrospective observational study Setting & Participants: SARS-CoV-2 vaccinated patients receiving maintenance dialysis through a national dialysis provider. Immunoglobulin G spike antibodies (anti-spike IgG) titers were assessed monthly following vaccination. Exposure(s): Two and three doses of SARS-CoV-2 vaccine Outcome(s): Undiagnosed and diagnosed SARS-CoV-2 infections; anti-spike IgG titers over time Analytical Approach: "Undiagnosed" SARS-CoV-2 infections were identified as an increase in anti-spike IgG titer of ≥100 BAU/mL, not associated with receipt of vaccine or "diagnosed" SARS-CoV-2 infection (by PCR or antigen test). In descriptive analyses, anti-spike IgG titers were followed over time. Results: Among 2660 patients without prior COVID-19 who received an initial two-dose vaccine series, 371 (76%) SARS-CoV-2 infections were diagnosed and 115 (24%) were undiagnosed. Among 1717 patients without prior COVID-19 who received a third vaccine dose, 155 (80%) SARS-CoV-2 infections were diagnosed and 39 (20%) were undiagnosed. In both cohorts, anti-spike IgG levels declined over time. Of the initial two-dose cohort, 66% had a titer ≥500 BAU/mL in the first month, with 23% maintaining a titer ≥500 BAU/mL at six months. Of the third dose cohort, 95% had a titer ≥500 BAU/mL in the first month after the third dose, with 76% maintaining a titer ≥500 BAU/mL at six months. Limitations: Assays used had upper limits. Conclusions: Among maintenance dialysis patients, 20-24% of SARS-CoV-2 infections were undiagnosed. Given this population's vulnerability to COVID-19, ongoing infection control measures are needed. A three-dose primary mRNA vaccine series optimizes seroresponse rate and durability.

The authors have declared no competing interest.

This report was supported by Dialysis Clinic, Inc. CMH has received support from ASN KidneyCure's Ben J. Lipps Research fellowship and from NIH/NCATS grant KL2TR002545. CMH's funders had no role in study design, data collection, reporting, or the decision to submit. Dr. Manley, Mr. Ladik, Dr. Frament, Dr. Johnson and Dr. Lacson Jr are all employees of DCI, where Dr. Johnson is Vice Chair of the Board. Dr. Weiner, Dr. Miskulin, Dr. Argyropoulos, Dr. Abreo, Dr. Chin, Dr. Gladish, and Dr. Salman receive salary support to their institution from DCI.

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This study was reviewed and approved by the WCG IRB Work Order 1-1456342-1.

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