Discussion

We found that kidney transplant recipients developed an antibody response following COVID-19; the mean antibody level and the seropositivity rate were similar to that of the control group. To the best of our knowledge, the largest report about antibody response in kidney transplant recipients was from Azzi et al. [9]. They examined 69 kidney transplant recipients who had an RT-PCR confirmed COVID-19 diagnosis, and 55 (80.0%) of them have positive antibody response. They used the same test as our study to measure antibody levels. They measured the antibody response following a median of 44 days following RT-PCR positivity. Hartzell et al. examined anti-SARS-Cov2 IgG antibodies in 16 kidney transplant recipients following a mean of 16.1 days of RT-PCR testing; antibody positivity rate was reported as 60.0% and 63.6% depending on immunosuppressive drug use [17]. Burack et al. examined 39 kidney transplant recipients and found an antibody positivity of 41.0% [7].
According to our data infection in a household member and high BMI was associated with COVID-19 infection in kidney transplant recipients. Previous studies showed an association between high BMI and severe COVID-19 [18]. However, it is not clear whether the risk of COVID-19 increases with high BMI [19-22]. A recent study showed that the prevalence of obesity in patients hospitalized with COVID-19 is higher than the worldwide prevalence of obesity [23].
In kidney transplant recipients who did not have positive RT-PCR testing, we found a similar SARS-CoV-2 IgG antibody positivity rate to our controls. In line with our results, a recent study reported that the seroprevalence rate was 6.6% in asymptomatic people [24]. In another study SARS-CoV-2 seroprevalence in healthy blood donors was reported as 3% [25].
We did not identify any specific risk factor for lack of seroconversion following COVID-19. However, we noted a trend toward lower antibody levels in patients who had a longer post-infection duration. A similar observation was reported by Benotmane et al. [26]. They examined 29 kidney transplant recipients hospitalized for COVID-19 and measured antibody levels up to 6 months after COVID-19. During the follow-up, 20.7% of the patients become seronegative. A considerable IgG reduction was observed in patients treated with calcineurin inhibitors and steroids. No statistically significant difference was found regarding disease severity.
Our study has some limitations; we did not formally screen all patients; we might have overlooked some patients who had severe COVID-19 and died. However, it is unlikely that we missed mild cases because most of the patients were in close telephone contact with transplant coordinators during this period. Another limitation of this study is that the date of infection was defined as the date of PCR positivity, as opposed to the date of symptom onset. Since transplant recipients may exhibit prolonged shedding of the virus, the date of PCR positivity may not always be an accurate estimate of infection onset. Kidney transplant recipients and controls had different distribution characteristics regarding duration between COVID-19 and antibody testing, all the controls were tested for antibodies following at least four weeks, and no control was tested following more than 12 weeks. However, the distribution of transplant recipients between different IgG testing duration categories was homogenous. Finally, we did not examine parameters related to cellular immunity.