Kidney transplant recipients (KTRs) are at increased risk for severe disease and death from COVID-19, and hence are prioritized for vaccination (1). Several studies evaluating the immune response following a two-dose mRNA vaccine schedule among KTRs demonstrated diminished humoral and cellular response (2–4). Seroconversion rates among KTRs receiving two doses of mRNA vaccine in these studies ranged from 36–54% compared to 100% in healthy controls (2–4). Similarly, T-cell response rates of 30–54% were demonstrated among KTRs, compared to over 95% among healthy controls (4,5). In addition, clinical cases of severe COVID-19, including fatal cases, were reported among fully (two-dose) vaccinated KTRs (6,7). The third mRNA vaccine dose has been recommended for severely immunocompromised patients since April 2021 in France, as well as by the European Medicines Agency (EMA) since October 2021 (1,8). Several previous studies evaluated the effectiveness and safety of a third mRNA vaccine dose among solid organ transplant (SOT) recipients (9–14), with three studies including solely KTRs (9,13,14). Humoral response among SOT recipients was demonstrated in 32–55% of those seronegative after two vaccine doses, without serious adverse events. Cellular response and predictors of negative immune response were partially evaluated (9–14). Immune response to two-dose mRNA vaccines varied between SOT types in previous studies, with KTRs being more responsive than lung transplant recipients, but less responsive than heart and liver transplant recipients (15–17).

In the current study, we aimed to evaluate humoral and cellular response specifically among KTRs ∼3 weeks after a third dose of BNT162b2 vaccine dose in Israel. We also aimed to identify variables associated with positive antibody response.

Of the 308 KTRs in the original cohort (2), 190 (61%) had a baseline anti-spike antibody test collected before the third vaccine dose, and were included in the current study. (See flow chart of patients’ selection in Supplementary Figure S1). Mean age was 59 years (SD 12), and 32% were females (61/190). Median time from third vaccination to antibody test collection was 29 days (IQR 20–33).

In this study including 190 KTRs, we found 70% seropositivity rates in response to a three dose BNT162b2 regimen, increasing from 37% after two doses. Twenty seven percent (52/190) achieved antibody response over 4160 AU/ml, associated with neutralization. T-cell response rate among KTRs was low, with 7/53 (13%) presenting adequate anti spike T-cell response. Variables associated with antibody response among KTRs included antibody levels after the second vaccine dose and immunosuppression reduction. The antibody response rate documented in our study is in accordance with recent studies reporting improved humoral response 28 days following a third mRNA dose in SOT recipients (9–14). Three of them including solely KTRs: Massa et al. reported antibody response in 32% of 61 seronegative KTRs, improving seropositivity rates from 44% to 62% after three BNT162b2 doses (13). Benotmane et al. reported 49% response among 159 previously seronegative KTRs after three doses of the mRNA-1273 vaccine (9). The relatively increased response rates in the latter study may represent a higher humoral immunogenicity with mRNA-1273 vaccine compared to BNT162b2, previously demonstrated in healthy people (23). Schrezenmeier et al. reported a 36% response rate among 25 previously seronegative KTRs, following either the third homologous BNT162b2 dose or heterologous ChAdOx1 (14). In SOT recipients in general, Kamar et al. reported a 44% response rate among 59 seronegative SOTs, most of them KTRs, with seropositivity rates increasing from 40% to 66% of 101 SOT recipients vaccinated with BNT162b2 (10). Finally, Hall et al. randomized 120 SOT recipients (29 KTRs, 25 kidney-pancreas), 10% seropositive at baseline, to either mRNA-1273 or placebo. Fifty five percent of 60 patients in the vaccine arm were seropositive after the third dose, compared with 18% in the placebo group (11). Magnitude of response was also demonstrated to increase after third dose, similar to our study (9,24).

Surprisingly, using the cutoff of 4160 AU/ml, the control group in our study was seronegative prior to the third dose. The mean age of participants in the control group was 69 years, and they were on average 200 days after the second vaccine. Waning of vaccine response has been demonstrated, mainly in older adults, which may explain the relatively low antibody titer (25). In addition, the cutoff used as surrogate for neutralization in our study was taken from previous studies. Additional studies may be needed to validate this cutoff as a surrogate for neutralization.

We found in our cohort 13% (7/53) of patients with adequate T-cell response, evaluated by SARS-CoV-2 spike-specific IFNg secretion. These rates are lower than the 30–60% cellular response rates reported after second vaccine dose among KTRs, measured by IFNg secreting cells frequency, which is of higher sensitivity than IFNg secretion assay (4). A significant increase in IFNg secreting cells was demonstrated in studies evaluating SOT recipients before and after the third vaccine dose (11,13,14). Percentage of responders was not reported in these studies, and assays differed, limiting our ability to compare them to our results. In a small study in cancer patients, no improvement in T-cell response was observed after the BNT162b2 third dose (26). Schrezenmeier et al. reported that KTRs with a humoral response to the third vaccine dose had significantly higher portions of antigen-reactive T cells than those without a humoral response (14).

Various predictors of antibody response to the third mRNA dose were reported from previous studies in SOT recipients. Among KTRs, use of antimetabolite, low lymphocyte count, and previous negative antibody response to the second vaccine dose predicted negative response (9). In SOT recipients in general, older age, higher degree of immunosuppression, and a lower estimated glomerular filtration rate were associated with no antibody response (10). Previous studies assessing immune response to second mRNA vaccine dose in KTRs demonstrated mycophenolate including regimen as strongly associated with low seroconversion rates (2,3). In the study by Schrezenmeier et al. only three individuals achieved high positive antibodies, one of them was the only person in the study without mycophenolate mofetil at the time of vaccination (14).

These findings may explain our results, showing immunosuppression reduction prior to vaccination to be associated with positive antibody response.

Following the third vaccine dose and although immunosuppression was discontinued for 14.2% of the cohort, no acute rejection episodes were found in our cohort. Previous studies demonstrated no serious adverse events and no acute rejection episodes among SOT recipients who received a third mRNA vaccine dose (9–11,24). In addition, previous studies have demonstrated no graft or patient survival impairment following temporary discontinuation of mycophenolate mofetil in KTRs treated with triple immunosuppression, as in our study (27,28).

Our study has several limitations. The sample size was limited with a small number of events included in the multivariable analysis. Cellular response was tested only after the third vaccine dose, with no previous testing after second dose for comparison. It was also assessed only in a limited subset of patients due to technical limitations. In addition, neutralizing antibodies were not directly assessed. Nevertheless, we used a cut-off of 4160 AU/ml of anti-spike antibodies as a surrogate, as previously described (21,22). Hall et al. demonstrated significant median percent virus neutralization (71%) with a third dose versus placebo (13%); and Massa et al. also demonstrated a significant increase in serum neutralizing activity between the second and third doses (11,13). The significance of higher antibody response as reflecting protection against infection and severe disease is still debated, though data are accumulating to support an association. A recent study from Israel demonstrated among healthcare workers both higher antibody levels and recused risk for infection after third versus second BNT162b2 dose. Greater incidence of infection was demonstrated among those with lower antibody levels (29). Immunosuppression reduction was not planned for the study and was initiated by either patients or treating physicians, due to reasons related or unrelated to the vaccine. This strategy should be tested in clinical trials, designated for this question (30).

Though our results and others present improved humoral immunity following three vaccine doses, still, at least 30% of KTRs remain seronegative, potentially susceptible to SARS-CoV-2 infection. These results support the administration of a third vaccine dose to KTRs; however, additional strategies should be discussed. These may include immunosuppression reduction prior to vaccination (30). Considering that anti-metabolites are consistently reported as associated with seronegative response, transient discontinuation of anti-metabolites prior to and shortly after the vaccine dose may be considered, monitoring meticulously for acute rejection (14). An alternative approach could be heterologous vaccination, i.e., using a non-mRNA vaccine for the third dose. Such a “mix and match” approach was recently supported by the EMA for the general population, stating that there are currently no data to support heterologous boosting among immunocompromised individuals (31). Two studies in KTRs used heterologous third dose boosting, with approximately a third to half of the recipients developing humoral response (12,14). This approach should be further tested in this population. An additional approach could be a fourth mRNA vaccine dose. Three studies evaluating a fourth vaccine dose among KTRs have been published so far, demonstrating improved immunogenicity, though with lower response rates among those still seronegative after the third dose (32–34). Otherwise, consideration of post-exposure or even pre-exposure prophylaxis could be an alternative to vaccination, using monoclonal antibodies or antiviral drugs (35–38). In addition, safety results should be verified in larger, longer term follow-up studies, including follow-up on rejection risk (39).

In summary, third dose BNT162b2 improves immune response over two doses in KTRs; however a significant percentage of these patients still remain seronegative and at risk for SARS-CoV-2 infection. Temporary withdrawal of the antimetabolite should be considered before the administration of the vaccine dose, taking into account individual risk for rejection. This strategy and others for improved immunization should be tested in clinical studies.