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S3. Boosted serological responses and persistent T cell responses following SARS-CoV-2 vaccination dose

This study offers longitudinal insight into the impact of three SARS-CoV-2 vaccinations on humoral and cellular immunity in patients with solid cancers, patients with hematologic malignancies, and persons without cancer. For all cohorts, virus-neutralizing immunity was significantly depleted over a period of up to 9 months following the second vaccine dose, the one striking exception being IL2 production by SARS-CoV-2 antigen-specific T cells. Immunity was restored by the third vaccine dose, except in a substantial number of patients with hematologic malignancy, for whom both cancer type and treatment schedule were associated with nonresponse. Thus, whereas most patients with myelodysplastic syndrome were conspicuously good responders, some patients with other hematologic malignancies receiving cancer therapies within 2 weeks of vaccination showed no seroconversion despite three vaccine doses. Moreover, SARS-CoV-2 exposure during the course of the study neither prevented immunity waning, even in healthy controls, nor guaranteed vaccine responsiveness. These data offer real-world human immunologic insights that can inform health policy for patients with cancer.Significance:Global health policy reliant on SARS-CoV-2 vaccine effectiveness is underpinned by our understanding of the durability of protection offered by sequential vaccinations and the efficacy of boosting, especially in immunocompromised patient populations who might constitute virus reservoirs. Here, we have: (i) clarified in patients with cancer the degree of waning of antibodies, serum neutralization titres against parental virus and variants of concern, and T-cell responses; (ii) evaluated the immune response among patients with cancer to a third dose of COVID-19 vaccine; and (iii) provided safety data following the third dose of the BNT162b2 COVID-19 vaccine in patients with cancer.

Supplementary Table & Figure Legends

Despite the success of COVID-19 vaccines in preventing infection and/or severe disease, with the emergence of SARS-CoV-2 variants of concern (VOC) which encode mutations in Spike, and the waning of vaccine induced immunity, there has been an increase in SARS-CoV-2 infections in vaccinated individuals which leads to increased serum neutralization breadth. However, how exposure to a heterologous Spike broadens the neutralizing response at the monoclonal antibody (mAb) level is not fully understood. Through isolation of 119 mAbs from three individuals receiving two-doses of BNT162b2 vaccine before becoming delta or omicron/BA.1-infected, we show that breadth arises from re-activation and maturation of B cells generated through previous COVID-19 vaccination rather than ade novoresponse specific to the VOC Spike. Isolated mAbs frequently show reduced neutralization of current circulating variants including BA.2.75.2, XBB, XBB.1.5 and BQ.1.1 confirming continuous selective pressure on Spike to evolve and evade neutralization. However, isolation of mAbs that display effective cross-neutralization against all variants indicate the presence of conserved epitopes on RBD and a lesser extent NTD. These findings have implications for selection of Spike antigens for next-generation COVID-19 vaccines.

This study offers longitudinal insight into the impact of three SARS-CoV-2 vaccinations on humoral and cellular immunity in patients with solid cancers, patients with hematologic malignancies, and persons without cancer. For all cohorts, virus-neutralizing immunity was significantly depleted over a period of up to 9 months following the second vaccine dose, the one striking exception being IL2 production by SARS-CoV-2 antigen-specific T cells. Immunity was restored by the third vaccine dose, except in a substantial number of patients with hematologic malignancy, for whom both cancer type and treatment schedule were associated with nonresponse. Thus, whereas most patients with myelodysplastic syndrome were conspicuously good responders, some patients with other hematologic malignancies receiving cancer therapies within 2 weeks of vaccination showed no seroconversion despite three vaccine doses. Moreover, SARS-CoV-2 exposure during the course of the study neither prevented immunity waning, even in healthy controls, nor guaranteed vaccine responsiveness. These data offer real-world human immunologic insights that can inform health policy for patients with cancer. Significance: Global health policy reliant on SARS-CoV-2 vaccine effectiveness is underpinned by our understanding of the durability of protection offered by sequential vaccinations and the efficacy of boosting, especially in immunocompromised patient populations who might constitute virus reservoirs. Here, we have: (i) clarified in patients with cancer the degree of waning of antibodies, serum neutralization titres against parental virus and variants of concern, and T-cell responses; (ii) evaluated the immune response among patients with cancer to a third dose of COVID-19 vaccine; and (iii) provided safety data following the third dose of the BNT162b2 COVID-19 vaccine in patients with cancer.

COVID-19 vaccine design and vaccination rollout need to take into account a detailed understanding of antibody durability and cross-neutralizing potential against SARS-CoV-2 and emerging variants of concern (VOCs). Analyses of convalescent sera provide unique insights into antibody longevity and cross-neutralizing activity induced by variant spike proteins, which are putative vaccine candidates. Using sera from 38 individuals infected in wave 1, we show that cross-neutralizing activity can be detected up to 305 days pos onset of symptoms, although sera were less potent against B.1.1.7 (Alpha) and B1.351 (Beta). Over time, despite a reduction in overall neutralization activity, differences in sera neutralization potency against SARS-CoV-2 and the Alpha and Beta variants decreased, which suggests that continued antibody maturation improves tolerance to spike mutations. We also compared the cross-neutralizing activity of wave 1 sera with sera from individuals infected with the Alpha, the Beta or the B.1.617.2 (Delta) variants up to 79 days post onset of symptoms. While these sera neutralize the infecting VOC and parental virus to similar levels, cross-neutralization of different SARS-CoV-2 VOC lineages is reduced. These findings will inform the optimization of vaccines to protect against SARS-CoV-2 variants., The authors assess the durability and long-term cross-reactivity of neutralizing antibodies raised in response to infections with SARS-CoV-2 or variants of concern in humans.

Summary Patients receiving targeted cancer treatments such as tyrosine kinase inhibitors (TKIs) have been classified in the clinically extremely vulnerable group to develop severe acute respiratory syndrome coronavirus‐2 (SARS‐CoV‐2), including patients with chronic myeloid leukaemia (CML) taking TKIs. In addition, concerns that immunocompromised individuals with solid and haematological malignancies may not mount an adequate immune response to a single dose of SARS‐CoV‐2 BNT162b2 (Pfizer‐BioNTech) vaccine have been raised. In the present study, we evaluated humoral and cellular immune responses after a first injection of BNT162b2 vaccine in 16 patients with CML. Seroconversion and cellular immune response before and after vaccination were assessed. By day 21 after vaccination, anti‐Spike immunoglobulin G was detected in 14/16 (87·5%) of the patients with CML and all developed a neutralising antibody response [serum dilution that inhibits 50% infection (ID50) >50], including medium (ID50 of 200–500) or high (ID50 of 501–2000) neutralising antibodies titres in nine of the 16 (56·25%) patients. T‐cell response was seen in 14/15 (93·3%) evaluable patients, with polyfunctional responses seen in 12/15 (80%) patients (polyfunctional CD4+ response nine of 15, polyfunctional CD8+ T‐cell response nine of 15). These data demonstrate the immunogenicity of a single dose of SARS‐CoV‐2 BNT162b2 vaccine in most patients with CML, with both neutralising antibodies and polyfunctional T‐cell responses seen in contrast to patients with solid tumour or lymphoid haematological malignancies.

The second and third years of the SARS-CoV-2 pandemic have been marked by the repeated emergence and replacement of ‘variants’ with genetic and phenotypic distance from the ancestral strains, the most recent examples being Delta and Omicron. Here we describe a hamster contact exposure challenge model to assess protection conferred by vaccination or prior infection against re-infection. We found that 2-doses of self-amplifying RNA vaccine based on the ancestral spike ameliorated weight loss following Delta infection and decreased viral loads, but had minimal effect on Omicron/BA.1 infection. Prior infection with ancestral or Alpha variant was partially protective against Omicron/BA.1 infection, whereas all animals previously infected with Delta and exposed to Omicron became infected, although shed less virus. We further tested whether prior infection with Omicron/BA.1 protected from re-infection with Delta or Omicron/BA.2. Omicron/BA.1 was protective against Omicron/BA.2, but not Delta reinfection, again showing Delta and Omicron have a very large antigenic distance. Indeed, cross-neutralisation assays with human antisera from otherwise immunonaïve individuals (unvaccinated and no known prior infection), confirmed a large antigenic distance between Delta and Omicron. Prior vaccination followed by Omicron or Delta breakthrough infection led to a higher degree of cross-reactivity to all tested variants. To conclude, cohorts whose only immune experience of COVID is Omicron/BA.1 infection may be particularly vulnerable to future circulation of Delta or Delta-like derivatives. In contrast, repeated exposure to antigenically distinct spikes, via infection and or vaccination drives a more cross-reactive immune response, both in hamsters and people.One Sentence SummaryInfection with the Delta and Omicron SARS-CoV-2 variants do not provide cross-protective immunity against reinfection with one another in hamsters.

Background: COVID-19 vaccines have robust immunogenicity in the general population. However, data for individuals with immune-mediated inflammatory diseases who are taking immunosuppressants remains scarce. Our previously published cohort study showed that methotrexate, but not targeted biologics, impaired functional humoral immunity to a single dose of COVID-19 vaccine BNT162b2 (Pfizer-BioNTech), whereas cellular responses were similar. Here, we aimed to assess immune responses following the second dose.Methods: In this longitudinal cohort study, we recruited individuals with psoriasis who were receiving methotrexate or targeted biological monotherapy (ie, tumour necrosis factor [TNF] inhibitors, interleukin [IL]-17 inhibitors, or IL-23 inhibitors) from a specialist psoriasis centre serving London and South-East England. The healthy control cohort were volunteers without psoriasis, not receiving immunosuppression. Immunogenicity was evaluated immediately before, on day 28 after the first BNT162b2 vaccination and on day 14 after the second dose (administered according to an extended interval regimen). Here, we report immune responses following the second dose. The primary outcomes were humoral immunity to the SARS-CoV-2 spike glycoprotein, defined as titres of total spike-specific IgG and of neutralising antibody to wild-type, alpha (B.1.1.7), and delta (B.1.617.2) SARS-CoV-2 variants, and cellular immunity defined as spike-specific T-cell responses (including numbers of cells producing interferon-γ, IL-2, IL-21).Findings: Between Jan 14 and April 4, 2021, 121 individuals were recruited, and data were available for 82 participants after the second vaccination. The study population included patients with psoriasis receiving methotrexate (n=14), TNF inhibitors (n=19), IL-17 inhibitors (n=14), IL-23 inhibitors (n=20), and 15 healthy controls, who had received both vaccine doses. The median age of the study population was 44 years (IQR 33-52), with 43 (52%) males and 71 (87%) participants of White ethnicity. All participants had detectable spike-specific antibodies following the second dose, and all groups (methotrexate, targeted biologics, and healthy controls) demonstrated similar neutralising antibody titres against wild-type, alpha, and delta variants. By contrast, a lower proportion of participants on methotrexate (eight [62%] of 13, 95% CI 32-86) and targeted biologics (37 [74%] of 50, 60-85; p=0·38) had detectable T-cell responses following the second vaccine dose, compared with controls (14 [100%] of 14, 77-100; p=0·022). There was no difference in the magnitude of T-cell responses between patients receiving methotrexate (median cytokine-secreting cells per 106 cells 160 [IQR 10-625]), targeted biologics (169 [25-503], p=0·56), and controls (185 [133-328], p=0·41).Interpretation: Functional humoral immunity (ie, neutralising antibody responses) at 14 days following a second dose of BNT162b2 was not impaired by methotrexate or targeted biologics. A proportion of patients on immunosuppression did not have detectable T-cell responses following the second dose. The longevity of vaccine-elicited antibody responses is unknown in this population.Funding: NIHR Biomedical Research Centre at Guy's and St Thomas' NHS Foundation Trust and King's College London; The Psoriasis Association.

Although the antibody response to COVID-19 vaccination has been studied extensively at the polyclonal level using immune sera, little has been reported on the antibody response at the monoclonal level. Here we isolate a panel of 44 anti-SARS-CoV-2 monoclonal antibodies (mAbs) from an individual who received two doses of the ChAdOx1 nCoV-19 (AZD1222) vaccine at a 12-week interval. We show that despite a relatively low serum neutralization titre, mAbs with potent neutralizing activity against the current SARS-CoV-2 variants of concern (B.1.1.7, P.1, B.1.351 and B.1.617.2) were obtained. The vaccine elicited neutralizing mAbs form 8 distinct competition groups and bind epitopes overlapping with neutralizing mAbs elicited following SARS-CoV-2 infection. AZD1222 elicited mAbs are more mutated than mAbs isolated from convalescent donors 1-2 months post infection. Spike reactive IgG+ B cells were still detectable 9-months post boost. These findings give molecular insights into AZD1222 elicited antibody response.

Although the antibody response to COVID-19 vaccination has been studied extensively at the polyclonal level using immune sera, little has been reported on the antibody response at the monoclonal level. Here, we isolate a panel of 44 anti-SARS-CoV-2 monoclonal antibodies (mAbs) from an individual who received two doses of the ChAdOx1 nCoV-19 (AZD1222) vaccine at a 12-week interval. We show that, despite a relatively low serum neutralization titer, Spike-reactive IgG+ B cells are still detectable 9 months post-boost. Furthermore, mAbs with potent neutralizing activity against the current SARS-CoV-2 variants of concern (Alpha, Gamma, Beta, Delta, and Omicron) are present. The vaccine-elicited neutralizing mAbs form eight distinct competition groups and bind epitopes overlapping with neutralizing mAbs elicited following SARS-CoV-2 infection. AZD1222-elicited mAbs are more mutated than mAbs isolated from convalescent donors 1-2 months post-infection. These findings provide molecular insights into the AZD1222 vaccine-elicited antibody response.

Summary Background Patients on therapeutic immunosuppressants for immune-mediated inflammatory diseases were excluded from COVID-19 vaccine trials. We therefore aimed to evaluate humoral and cellular immune responses to COVID-19 vaccine BNT162b2 (Pfizer-BioNTech) in patients taking methotrexate and commonly used targeted biological therapies, compared with healthy controls. Given the roll-out of extended interval vaccination programmes to maximise population coverage, we present findings after the first dose. Methods In this cohort study, we recruited consecutive patients with a dermatologist-confirmed diagnosis of psoriasis who were receiving methotrexate or targeted biological monotherapy (tumour necrosis factor [TNF] inhibitors, interleukin [IL]-17 inhibitors, or IL-23 inhibitors) from a specialist psoriasis centre serving London and South East England. Consecutive volunteers without psoriasis and not receiving systemic immunosuppression who presented for vaccination at Guy's and St Thomas' NHS Foundation Trust (London, UK) were included as the healthy control cohort. All participants had to be eligible to receive the BNT162b2 vaccine. Immunogenicity was evaluated immediately before and on day 28 (±2 days) after vaccination. The primary outcomes were humoral immunity to the SARS-CoV-2 spike glycoprotein, defined as neutralising antibody responses to wild-type SARS-CoV-2, and spike-specific T-cell responses (including interferon-γ, IL-2, and IL-21) 28 days after vaccination. Findings Between Jan 14 and April 4, 2021, 84 patients with psoriasis (17 on methotrexate, 27 on TNF inhibitors, 15 on IL-17 inhibitors, and 25 on IL-23 inhibitors) and 17 healthy controls were included. The study population had a median age of 43 years (IQR 31–52), with 56 (55%) males, 45 (45%) females, and 85 (84%) participants of White ethnicity. Seroconversion rates were lower in patients receiving immunosuppressants (60 [78%; 95% CI 67–87] of 77) than in controls (17 [100%; 80–100] of 17), with the lowest rate in those receiving methotrexate (seven [47%; 21–73] of 15). Neutralising activity against wild-type SARS-CoV-2 was significantly lower in patients receiving methotrexate (median 50% inhibitory dilution 129 [IQR 40–236]) than in controls (317 [213–487], p=0·0032), but was preserved in those receiving targeted biologics (269 [141–418]). Neutralising titres against the B.1.1.7 variant were similarly low in all participants. Cellular immune responses were induced in all groups, and were not attenuated in patients receiving methotrexate or targeted biologics compared with controls. Interpretation Functional humoral immunity to a single dose of BNT162b2 is impaired by methotrexate but not by targeted biologics, whereas cellular responses are preserved. Seroconversion alone might not adequately reflect vaccine immunogenicity in individuals with immune-mediated inflammatory diseases receiving therapeutic immunosuppression. Real-world pharmacovigilance studies will determine how these findings reflect clinical effectiveness. Funding UK National Institute for Health Research.

As SARS-CoV-2 variants continue to emerge globally, a major challenge for COVID-19 vaccination is the generation of a durable antibody response with cross-neutralizing activity against both current and newly emerging viral variants. Cross-neutralizing activity against major variants of concern (B.1.1.7, P.1 and B.1.351) has been observed following vaccination, albeit at a reduced potency, but whether vaccines based on the Spike glycoprotein of these viral variants will produce a superior cross-neutralizing antibody response has not been fully investigated. Here, we used sera from individuals infected in wave 1 in the UK to study the long-term cross-neutralization up to 10 months post onset of symptoms (POS), as well as sera from individuals infected with the B.1.1.7 variant to compare cross-neutralizing activity profiles. We show that neutralizing antibodies with cross-neutralizing activity can be detected from wave 1 up to 10 months POS. Although neutralization of B.1.1.7 and B.1.351 is lower, the difference in neutralization potency decreases at later timepoints suggesting continued antibody maturation and improved tolerance to Spike mutations. Interestingly, we found that B.1.1.7 infection also generates a cross-neutralizing antibody response, which, although still less potent against B.1.351, can neutralize parental wave 1 virus to a similar degree as B.1.1.7. These findings have implications for the optimization of vaccines that protect against newly emerging viral variants.

Encouraging results have been observed from initial studies evaluating vaccines targeting the novel beta coronavirus which causes severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, concerns have been raised around the efficacy of these vaccines in immunosuppressed populations, including patients with haematological malignancy. Myeloproliferative neoplasms (MPN), in particular myelofibrosis (MF), are associated with heterogenous immune defects which are influenced by patient age, disease subtype and the use of cytoreductive therapies. Patients with a WHO defined diagnosis of an MPN presenting to our clinic were recruited following first injection of 30μg BNT162b2. A positive anti-S IgG ELISA was seen in 76.1% (16) of patients following vaccination with positive neutralising antibodies detected in 85.7% (18) of patients. A memory T cell response was observed in 80% (16) of patients, with a CD4+ T cell response in 75% (15) and a CD8+ T cell response in 35% (7). These results, for the first time, provide some reassurance regarding the initial immune response to the BNT162b2 vaccine amongst patients with MPN, with response rates similar to that observed in the general population.

Patients receiving targeted cancer treatments such as tyrosine kinase inhibitors (TKIs) have been classified in the clinically extremely vulnerable group to develop severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), including patients with Chronic Myeloid Leukaemia (CML) taking TKIs. In addition, concerns that immunocompromised individuals with solid and haematological malignancies may not mount an adequate immune response to a single dose of SARS-CoV-2 BNT162b2 (Pfizer-BioNTech) vaccine have been raised.We evaluated humoral and cellular immune responses after a first injection of BNT162b2 vaccine in 16 CML patients. Seroconversion and cellular immune response prior and after vaccination were assessed.By day 21 post-vaccination, anti-Spike IgG were detected in 14/16 (87.5%) of CML patients and all developed a neutralizing antibody response (ID50>50), including medium (ID50 of 200-500) or high (501-2000) neutralising antibodies titres in 9/16 (56.25%) patients. T cell response was seen in 14/15 (93.3%) evaluable patients, with polyfunctional responses seen in 12/15 (80%) patients (polyfunctional CD4+ response 9/15, polyfunctional CD8+ T cell response 9/15).These data demonstrate the immunogenicity of a single dose of SARS-CoV-2 BNT162b2 vaccine in most CML patients with both neutralizing antibodies and polyfunctional T-cell responses seen, in contrast to patients with solid tumour or lymphoid haematological malignancies.FundingKing’s Together Rapid COVID-19 Call awards to MHM, KJD; A Huo Family Foundation Award to MHM, KJD; Chronic Disease Research Foundation award CDRF-22/2020 to KJD, MHM; Wellcome Trust Investigator Award 106223/Z/14/Z to MHM; CG was supported by the MRC-KCL Doctoral Training Partnership in Biomedical Sciences (MR/N013700/1); Fondation Dormeur, Vaduz for funding equipment to KJD

Background: Cancer patients are vulnerable populations for COVID-19 complications and mortality. We previously reported on the poor single-dose immunogenicity of BNT162b2 mRNA vaccine in cancer patients, particularly those with haematological malignancies. Methods: In this prospective, observational study relating to the safety and immunogenicity of BNT162b2 mRNA vaccine, 201 vaccinated cancer patients (solid cancer n=125; haematological cancer n=76) and 54 healthy controls (mostly health-care workers “HCW”) were recruited between December 8th, 2020, and April 23rd, 2021. The previously reported interim results covered a period of 101 days since first patient recruitment, during which time 47 subjects received a second “boost” vaccination on day 21. Because of the change in UK Government policy, all others received a delayed vaccine boost at about 12 weeks after their first vaccination, and had their blood sampled 2 weeks’ later. Here, we describe immunogenicity data following the delayed boost in 31 HCWs, 72 solid cancer and 56 haematological cancer patients. Seroconversion, virus neutralisation, and T cell assays were as described previously, with an additional test for neutralisation of the B.1.617.2 (delta) variant-ofconcern (VOC). The primary endpoint of the study was the impact on seroconversion following delayed (>21days) vaccine boosting in solid and haematological cancer patients. The secondary endpoints were: safety following delayed vaccine boost; T cell responses; and neutralisation of SARS-CoV-2 Wuhan (“wild type” [WT]), B.1.1.7 (alpha), and B.1.617.2 (delta) variants. Findings: Delayed (>21days) boost vaccination of solid cancer patients and haematological cancer patients with the BNT162b2 vaccine was well tolerated, as the primary vaccination had been. There was no vaccine-associated death. Boosting significantly increased solid cancer patients’ seroconversion responses, that had been strikingly poor in response to a single dose: from 38% to 84%. Boosting also significantly improved vaccine immunogenicity for haematological cancer patients, but most (57%) still failed to seroconvert. Seroconversion correlated strongly with the capacity to neutralise SARSCoV- 2 cell entry, although neutralisation of the WT variant was typically greater than of the VOC. Neutralisation was significantly increased by boosting for HCWs but not for cancer patients. In comparison to seroconversion, boosting achieved higher rates of functional T cell responsiveness (de novo responses) but had little impact on the magnitude of T cell responses for those who had responded to first-dose vaccination. When patients were scored as showing both seroconversion and T cell responses, the unfavourable situation of haematological cancer patients was overt with only 36% (12/33) defined as being responders compared to 78% (25/32) of solid cancer patients and 88% (15/17) of HCWs. There was no significant difference in any aspect of immunogenicity for HCWs or solid cancer patients receiving the delayed boost versus the day 21 boost (this comparison could not be made for haematological cancer patients because too few received an early boost). Chemotherapy within 15 days either side of the boost exacerbated the likelihood of non-responsiveness to the vaccine. Interpretation: Boosting at either 3 weeks or longer (up to 12 weeks) post-primary vaccination shows high efficacy in terms of seroconversion of solid cancer patients and increases in their SARS-CoV-2 Spike-specific antibody titres. By contrast, delayed boosting left most haematological cancer patients without serological protection against SARS-CoV-2 infection. These data support the ongoing adjustment of health care measures to limit the evident vulnerability of such individuals to SARS-CoV- 2, and to limit their potential to transmit virus variants that might develop in the context of absent or partial immunoprotection. The absence of any clear improvements in immunogenicity of a delayed boost relative to boosting on day 21 emphasizes the importance of early boosting for cancer patients, and potentially of doing so repeatedly, particularly given how well the vaccine was tolerated. Chemotherapy, if possible should be withheld 15 days before and 15 days after the vaccination date. Trial Registration: The trial is registered with the NHS Health Research Authority (HRA) and Health and Care Research Wales (HCRW) (REC ID: 20/HRA/2031). Funding: KCL, CRUK, Leukemia & Lymphoma Society, Wellcome Trust, Rosetrees Trust, Francis Crick Institute. Declaration of Interest: None to declare. Ethical Approval: The trial was approved by the institutional review boards of the participating institutions (IRAS ID: 282337 REC ID: 20/HRA/2031).

Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has led to unprecedented healthcare challenges on a global scale. Impressive efforts have led to rapid development of multiple efficacious vaccines against SARS-CoV-2, however concerns remain over the degree of protection vaccination offers to immunocompromised recipients. To answer this question, we have designed a prospective study to evaluate response to vaccination in patients with haematological malignancies (Harrington, Leukaemia 2021; Harrington, BJHaem, 2021). 103 patients were included with samples collected in 60 patients after first dose and 71 patients following second dose. We have analysed humoral and T cell response to a first dose of vaccine against SARS-CoV-2 in patients post allogeneic stem cell transplantation (HSCT) and compared those to patients with CML or MPN. Methods: ELISA plates were coated with antigen Nuclear (N) protein or the S protein. Serial dilutions of plasma were added to wells and incubated for 2 h at room temperature. Control reagents included N-specific monoclonal antibody, S-specific monoclonal antibody, negative control plasma, positive control plasma and blank wells. Secondary antibody was added and incubated for 1h at room temperature. IgG was detected using goat-anti-human-Fc-AP and plates read at 405 nm. Where an EC50 was not reached at 1:25, a plasma was considered seropositive if the OD at 405nm was 4-fold above background and a value of 25 was assigned. T cell functionality was assessed using intracellular cytokine staining after incubation with SARS-CoV-2 specific peptides covering immunogenic domains of the Spike (S) protein. A response was considered positive if there was a 3-fold increase in pro-inflammatory cytokine expression from baseline, and above a threshold of 0.01. Specific peptides (0.25 µg/ml), anti-CD28 and BFA were added to cells. Unstimulated cells were utilised as negative controls. Cells were stained with viability dye, then with antibodies directed against surface markers, and fixed and permeabilised prior to staining for intracellular cytokines TNFa and IFNg. Gating on lymphocytes, single cells, live cells, CD3+ cells, CD4+ cells and CD4- (CD8+) was performed. Results: Of the 103 patients included in this study, post vaccination evaluation on 56 patients have been analysed so far, including 37 patients with chronic myeloid malignancies (MPN n=21 and CML n=16) and 19 patients post HSCT. From the latter group, median time since transplant was 53.9 months (18.7 to 76.8) with 12 participants on extracorporeal photopheresis (ECP) therapy for graft versus host disease (GvHD) with median frequency of 24.5 days (14-42). BNT162b2 vaccine was administered to 48 patients (85.7%). An anti-S IgG response was observed after a first dose in 16/21 (76.1%) of the MPN group and 14/16 (87.5%) of CML patients, but in only 7/19 (36.8%) of post HSCT patients (Fishers Exact Test - p=0.02/0.002, Fig 1a). Of the latter group a low positive value where an EC50 was not reached was observed in 4/19 (21.1%) and a moderate response in 3/19 patients (15.8%). Of the 12 patients with active GvHD on ECP, a positive response was observed in 4 patients (33.3%), however only one patient recorded a response where an EC50 was measurable. A T cell response was observed in 16/20 (80%) of the MPN group and 14/15 (93.3%) of those with CML after a single dose, with a polyfunctional T cell response (>1 cytokine) observed in 65% and 80% respectively. In comparison only 5/19 patients (38.5%) post HSCT mounted a T cell response (p=0.027/p=0.002, Fig 1b), with a CD4+ response in 4 (30.8%) and a CD8+ response in 3 (23.1%). In this group, a polyfunctional T cell response was found in 4/19 patients (30.8%). 33.3% of patients with GVHD requiring ECP had a T cell response, compared with 42.9% in post HSCT without GVHD. Summary: Despite encouraging results of antibody and T cell response to a first vaccination dose in patients with chronic myeloid malignancies, these results raise concerns regarding the humoral and T cell responses to vaccination in patients post HSCT, recognised as a particularly immunosuppressed group. Further longitudinal data is required to determine if these results translate into a reduction in cases and severity of infection in these groups. We are currently analysing the response to a second vaccine injection and responses to sequential doses of vaccination across the whole cohort will be presented. Figure 1 Figure 1. Disclosures Harrington: Bristol Myers Squibb: Research Funding; Incyte: Honoraria. Radia: Blueprint Medicines Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Study steering group member, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Education events; Cogent Biosciences Incorporated: Other: Study Steering Committee; EXPLORER and PATHFINDER studies: Other: Member of the Response Adjudication Committee. Kordasti: Beckman Coulter: Honoraria; Celgene: Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Alexion: Honoraria. Dillon: Menarini: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Session chair (paid to institution), Speakers Bureau; Astellas: Consultancy, Other: Educational Events , Speakers Bureau; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Research Support, Educational Events; Amgen: Other: Research support (paid to institution); Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: educational events; Jazz: Other: Education events; Shattuck Labs: Membership on an entity's Board of Directors or advisory committees. Harrison: Promedior: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; AOP Orphan Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte Corporation: Speakers Bureau; Gilead Sciences: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Constellation Pharmaceuticals: Research Funding; Galacteo: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Geron: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sierra Oncology: Honoraria; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; CTI BioPharma: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Keros: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. de Lavallade: Bristol Myers Squibb: Research Funding; Incyte: Honoraria, Research Funding; Novartis: Speakers Bureau.