SARS-CoV-2 vaccines induce humoral and cellular immune responses [1, 2], protecting against infection, severe disease, and death [1]. Which compartment bears greater relevance for protection is unclear. Most vaccinees are not tested for humoral, let alone cellular responses; most healthy individuals will respond with both [2-4]. Clinical experience confirms an excessive risk of immunocompromised patients for severe COVID-19 [5-7], giving them priority access to vaccination, although suboptimal responses were predicted. Humoral responses are easily detected, and cellular responses are less so [8]. Hematopoietic stem cell transplantation (HSCT) recipients remain immunocompromised after numeric T-cell recovery and immunosuppressant withdrawal [9]. In contrast to healthy SARS-CoV-2 vaccinees, immunocompromised patients’ vaccine responses should be monitored to identify failure to develop protection [2]. In a cohort of vaccinated, nonseroconverted HSCT patients, induction of S1 domain of spike protein (S1-) specific T-cell responses was assessed to distinguish isolated B cell from combined adaptive immune incompetence.

Vaccination was initiated ≥3 months post-HSCT. Adult post-HSCT patients after two doses of Comirnaty (Biontec, Mainz, D) or Vaxzevria (Astra-Zeneca, Gothenburg, S) were monitored for anti-Spike-antibodies. Nonseroconverters beyond week 3 could participate in this study. B-non-Hodgkin's lymphoma (B-NHL) patients with isolated pharmacological B-cell depletion and healthy vaccinees served as controls. A conventional commercial in vitro T-cell stimulation assay determined T-cell response to S1-peptide (Supporting Information Methods).

Of 152 double-vaccinated patients, 27 (17.8%) developed no antibodies; 17 thereof (13 2xComirnaty, 3 2xVaxzevria, 1 1xheterologous) participated in our study. At the time of vaccination, 16 of 17 patients were >6 (median 47, range 5–1409) months out from HSCT and were full donor-type chimeras with adequate graft function.

Peripheral blood was analyzed for lymphocyte subpopulations and SARS-CoV-2-specific T cells for 55 days (median; range, 21–127) after the second vaccination. B cells were detected or normal in 14 or 10 of 17 patients. CD8⁺ T cells were at least normal in 16 of 17 patients, and absolute CD4⁺ T-cell lymphopenia was prevalent (14/17). Even the three patients with low-normal CD4⁺ cell counts had skewed CD4:CD8 ratios (Fig. 1A and B). Patients’ response to the TCR-MHC-cross-linking reagent was normal (n.s.) [9]; SARS-CoV-2 vaccination-specific responses, however, were diminished (Fig. 1C and D). Patients 2, 4, and 1 demonstrated isolated helper T-cell, cytotoxic T-cell, and combined T-cell responses; the total probability of SARS-CoV-2-specific T-cell responses was 7/17 (41%), 5/13, and 2/3 after 2xComirnaty and 2xVaxzevria. All patients with iatrogenic B-cell aplasia were responders. Unifying features of the remaining responders were not apparent. Thus, the absence of humoral responses does not preclude posttransplant cellular vaccination responses.

We next tested specifically B-cell depleted, inherently seronegative B-NHL patients (Table 1); responses of CD4⁺ and CD8⁺ T cells were observed in five of five and four of five (Fig. 1D). Of 22 healthy, antibody-positive vaccinees, 20 of 22 and 16 of 22 had antigen-responsive CD4⁺ and CD8⁺ cells. Only one had no T-cell response (Fig. 1D), in agreement with published data [8]. Of patients receiving systemic immunosuppressive therapy (“IS”) or not (“no IS”) concurrent to vaccination, four of 12 and three of five generated spike-protein-specific T cells (χ², p = 0.54/0.12 for CD4⁺/CD8⁺ T cells; Fig. 1D). T-cell responses were less frequent in post-HSCT patients, but, where observed, were of normal magnitude.

COVID-19-related mortality for patients with hematological cancer dramatically exceeds that for the general population including the elderly [5-7]. The effectiveness of SARS-CoV-2 vaccines in such vulnerable cohorts was not robustly established.

As shown here for the SARS-CoV-2 vaccine, adaptive immunity after HSCT often remains insufficient even after the withdrawal of immunosuppressants. A total of 17.8% of post-HSCT patients generated no vaccine antibodies. The majority thereof additionally did not develop T-cell responses, remaining unprotected against the SARS-CoV-2 virus.

The concurrently performed positive control, CytoStim, demonstrates patients’ overall unimpaired T-cell responsiveness, while sensitization to new antigen (spike protein) is deficient. After HSCT, low CD4:CD8 ratios indicate incomplete T-cell reconstitution with defective thymic recovery of CD4⁺ and predominant homeostatic expansion of peripheral CD8⁺ T cells. Limited T-cell receptor diversity precludes adequate vaccination responsiveness [9].

No systemic immunosuppressive therapy (“no IS”) patients and patients receiving systemic immunosuppressive therapy (“IS”) among the humoral non-responders were not relevantly different re. lymphocyte counts or CD4:CD8 ratios. Nonseroconverting “no IS” were also no more likely than “IS” patients to mount S1-specific T-cell responses (3/5 vs. 4/12, n.s.). Our data thus confirm and expand on recent reports in a similar cohort [10].

It is difficult to know if isolated T-cell responses provide protection against SARS-CoV-2. The 59% of our cohort (12% of the total posttransplant cohort) mounting neither humoral nor cellular responses presumably remain immunologically unprotected and should be counseled accordingly.

Besides the small sample size, a shortcoming of our analysis is its limitation to nonseroconverters. Possible divergence of T- and B-cell responses would be interesting but was not covered by the protocol. Profound lymphopenia precluded deeper phenotyping of (responding) T cells.

We conclude that the majority of allogeneic HSCT patients not showing humoral responses to SARS-CoV-2 vaccination also fail to mount antigen-specific T-cell responses. Immunosuppressive treatment postallogeneic HSCT does not preclude vaccination responses. High-nonresponse rates of HSCT patients to SARS-CoV-2 vaccination mandate testing for humoral and cellular responses to provide tailored guidance.

Conflict of interest

The authors declare no commercial or financial conflict of interest.