Serial SARS-CoV-2 Antibody Titers in Vaccinated Dialysis Patients: Prevalence of Unrecognized Infection and Duration of Seroresponse

Rationale & Objective: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections are likely underdiagnosed, but the degree of underdiagnosis among patients receiving maintenance dialysis is unknown. The durability of the immune response after the third vaccine dose in this population also remains uncertain. This descriptive study tracked antibody levels to (1) assess the rate of undiagnosed infections and (2) characterize seroresponse durability after the third dose.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 immunoglobulin (Ig) G] titers were assessed monthly after vaccination.Exposures: Two and 3 doses of SARS-CoV-2 vaccine.Outcomes: 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 polymerase chain reaction test or antigen test). In descriptive analyses, anti-spike IgG titers were followed over time.Results: Among 2,703 patients without previous coronavirus disease 2019 (COVID-19) who received an initial 2-dose vaccine series, 271 had diagnosed SARS-CoV-2 infections (3.4 per 10,000 patient-days) and 129 had undiagnosed SARS-CoV-2 infections (1.6 per 10,000 patient-days). Among 1,894 patients without previous COVID-19 who received a third vaccine dose, 316 had diagnosed SARS- CoV-2 infections (7.0 per 10,000 patient-days) and 173 had undiagnosed SARS-CoV-2 infections (3.8 per 10,000 patient-days). In both cohorts, anti-spike IgG levels declined over time. Of the initial 2-dose cohort, 66% had a titer of >= 500 BAU/mL in the first month, with 24% maintaining a titer of >= 500 BAU/mL at 6 months. Of the third dose cohort, 95% had a titer of >= 500 BAU/mL in the first month after the third dose, with 77% maintaining a titer of >= 500 BAU/mL at 6 months.Limitations: The assays used had upper limits.Conclusions: Among patients receiving mainte- nance dialysis, about 1 in every 3 SARS-CoV-2 infections was undiagnosed. Given this population's vulnerability to COVID-19, ongoing infection control measures are needed. A 3-dose primary mRNA vaccine series optimizes seroresponse rate and durability.