Results

Demographic, Clinical and Laboratory data

We recruited a total of 91 kidney transplant recipients, 46 of them had an RT-PCR confirmed COVID-19, and 45 of them did not have COVID-19 history (Table 1). Demographic, clinical, and laboratory data of transplant recipients grouped according to COVID-19 status are shown in Table 1. Both groups were similar regarding age and sex. Etiology of CKD and donor type and post-transplant duration were also similar between the two groups. However, patients with COVID-19 had a higher BMI, and COVID-19 history in a household member was more common among them. Other parameters were similar between the two groups.
The majority (95.6%) of the patients with COVID-19 were symptomatic, and according to thorax computed tomography examination, 30 (65.2%) patients had findings compatible with COVID-19. There was no need for hospitalization in 12 (26.1%) patients; the remaining 34 (73.9%) patients were hospitalized, 16 (34.8%) patients needed oxygen, and three (6.5%) of them were followed up in the intensive care unit. Two (4.3%) patients needed intubation. Except for one patient who died on the 26th day of the infection, all patients recovered from COVID-19. The hospitalization duration was 11.7±7.9 days (median:9 days, range:3-38 days). According to the COVID-19 severity index, two (4.3%) patients were asymptomatic or presymptomatic, 15 (32.6%) patients had a mild illness, 16 (34.7%) patients had a moderate illness, 10 (21.7%) patients had a severe illness, and three (6.5%) patients had a critical illness.

Seropositivity

Seropositivity rate and IgG levels among kidney transplant recipients and controls stratified by COVID-19 status are shown in Table 2. Among subjects with COVID-19 history, the SARS-Cov-2 IgG positivity rate and IgG level of kidney transplant recipients were similar to that of the control group. The frequency of COVID-19 related symptoms was more common among kidney transplant recipients than that of the controls; however, the frequency of pulmonary involvement assessed by computerized tomography was similar between the two groups. There was no statistically significant difference between the kidney transplant recipients and controls in terms of the duration from RT-PCR to antibody testing. (Table 2).
Among subjects without COVID-19 history, three kidney transplant recipients had positive IgG antibodies, and two of them had a history of COVID-19 in a household member. SARS-Cov-2 IgG antibody positivity rate and IgG level of kidney transplant recipients were similar to that of the control group (Table 2).

Predictors of antibody positivity

We compared demographic, clinical, laboratory, and treatment-related data of the transplant patients who developed antibodies with those who did not (Table 3). The median duration between RT-PCR and antibody testing was shorter (37.5 [IQR 20.5-57.8] vs 82.5 [IQR 52.3-105.0] days, p=0.01) in patients who had SARS-Cov-2 IgG antibodies compared to that of the patients who do not have IgG antibodies. There were no statistically significant differences among the two groups regarding demographic, clinical, laboratory parameters. Additionally, the cessation rate of different immunosuppressive drugs was also similar between the two groups.
As an additional analysis, we looked at the correlation between different laboratory parameters (peak CRP, peak ferritin, fibrinogen, peak d-dimer, peak procalcitonin, e-GFR) and level of SARS-Cov-2 IgG antibodies. There was no significant correlation between those parameters (data not shown).
Finally, we analyzed the correlation between SARS-Cov-2 IgG antibody levels and the duration between RT-PCR and antibody testing. The antibody level in kidney transplant recipients and controls according to duration following RT-PCR testing is shown in Figure 1. Visual examination revealed that there was a trend toward lower antibody levels in kidney transplant recipients with increasing post-infection duration. There was a significant correlation between antibody level and duration in transplant recipients (r=-0.532, p<0.001), however, there was no statistically significant correlation between antibody level and duration in controls (r=0.198, p=0.186). Additionally, we constructed a multivariate regression model; we used antibody levels as the dependent variable, and the study group along with the duration following RT-PCR testing as the independent variables. This analysis showed that the study group was not an independent determinant of antibody levels (data not shown).