Background: Since the beginning of the pandemic, children's role in the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been debated. We aimed to describe the prevalence of SARSCoV-2 in asymptomatic children undergoing institutional systematic screening. Methods: From 2020 to 2021, this retrospective study in a French university hospital included consecutive asymptomatic children routinely screened for SARS-CoV-2 infection by polymerase chain reaction (PCR) assay before surgery. Results: Among the 816 test samples, the prevalence of positive PCR results was 0.49 % (95 % CI: 0.01-0.97, n = 4); half of the cases involved close contacts with an adult case. Conclusion: These results support the low prevalence of SARS-CoV-2 in asymptomatic children during the first pandemic periods in France. (c) 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.

Introduction: Data regarding the efficacy of anti-severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) messenger RNA vaccines in immunocompromised hosts are scarce and no data yet appear to be available for patients with hematological malignancies who also received chimeric antigen receptor-T (CAR-T) cells therapy. Methods: The efficacy and safety of one and/or two injections of the BNT162b2 (Pfizer-BioNTech) vaccine was evaluated retrospectively in 23 CAR-T recipients in our Hematology Department, compared to a cohort of 25 healthy caregivers, vaccinated concomitantly between January 28 and May 31, 2021. None of these individuals had a previous clinical history of COVID-19. Results: Overall, the patients (14 males and 9 females) had a median age of 62 years old (range: 21-79) and had received CAR-T for high-grade lymphoma (n= 20) or acute lymphoblastic leukemia (n= 3). Eight and 3 had been respectively previously autografted or allografted and two were allografted after CAR-T. All patients were pretreated for lymphodepletion by fludarabine + cyclophosphamide before CAR-T infusion. The CAR-T provided were axicabtagene ciloleucel (Yescarta, Kite/Gilead, n=16, tisagenlecleucel (Kymriah, Novartis Pharma, n=5 and brexucabtagene autoleucel (KTE-X19, Tecartus, Kite/Gilead, n=1). One additional patient received allogeneic UCART19 (Servier). The median delay between CAR-T administration and the first vaccine (V1) was 401 days (d; range: 113-819). All patients but 2 were in complete remission at V1 and 3 were still on therapy (revlimid n=1, tafasitamab n=1, chemotherapy n=1). After V1, antibody response to the SARS-CoV-2 spike protein receptor-binding domain was tested by the Roche Elecsys® assay at a median time of 29 d (range: 16-32) in 19 patients and 23 d (range:18-32) in controls. At that time, only 4/23 patients (21%) but all controls (100%) had a positive anti-spike antibody response (p<0.001). Among seropositive cases, median IgG titers were higher in controls (35.1 U/mL, range 2.2->250) than in patients (5.9 U/mL range 4.1-41.6, p=0.06). The highest IgG titer (>250) was obtained in 2 controls. The median delay between V1 and the second vaccine (V2) was 28 d (range: 14-46) for patients and 23 d (range: 18-32) for controls. Among the 20 patients tested after V2, 17 had also been tested after V1 while 3 were tested only after V2. All controls were tested after V2. The second serology assay was performed at a median interval from V2 of 52 d (range: 21-99) for patients and 58 d (range: 32-71) for controls. This serology assay was positive in 6 patients (30%), while all controls (100%, p<0.001) had again a positive response. Three out of these 6 patients (15% of all patients) achieved the highest IgG titer according to the serology assay used. Among the 4 patients with positive antibody titers after V1, 3 remained positive including one reaching the highest IgG titer. The fourth patient has not yet received V2. Median IgG titers could not be compared with controls because various methods of detection were used after V2. However, all controls tested again by Roche Elecsys® displayed the highest IgG titer (>250) after V2. The two patients in relapse and treated by chemotherapy or tafasitamab did not develop antibodies after V2 conversely to the patient under maintenance by revlimid. The delay between CAR-T infusion and vaccine did not influence the antibody response nor did the rate of lymphopenia as almost all patients remained under a lymphocyte threshold of 1x10 9/L. Finally, with a median follow up from V1 of 77 d (range: 49-127) in patients and 81 d (range: 62-95) in controls, no COVID-19 infection has been documented in any of these participants. Conclusion: This study shows that the administration of two doses of BNT162b2 anti-SARS-CoV-2 messenger RNA vaccine provides a low rate of seroconversion (30%) in recipients of CAR-T therapy. This is likely related to the profound B-cell depletion induced by this treatment precisely targeting CD19+ cells. Investigation of the development of specific T-cell responses in these individuals could provide more information about the efficacy of vaccination in this context. Disclosures Moreau: Celgene BMS: Honoraria; Sanofi: Honoraria; Janssen: Honoraria; Abbvie: Honoraria; Amgen: Honoraria; Oncopeptides: Honoraria.

Abstract Background Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection results in poor outcome in patients with hematologic malignancies. Moreover, the efficacy of anti-SARS-CoV-2 mRNA vaccines appears lower in immunocompromised patients, including recipients of allogeneic stem cell transplantation (Allo-HSCT). In this population, data are scarce regarding factors predicting the response to mRNA vaccines. Methods This retrospective study aimed to decipher which factors, including immune status at time of vaccine and recipient/donor blood groups, might influence the antibody response after two injections (V1 and V2) of BNT162b2 (Pfizer-BioNTech) vaccine in a cohort of allografted patients with no previous symptomatic nor asymptomatic COVID-19 infection. Possible previous asymptomatic COVID-19 infection was investigated in pre-V1 samples by testing for anti-nucleocapsid (N) antibodies (anti-SARS-CoV-2 immunoassay, Roche Elecsys®, Rotkreuz, Switzerland). Antibody response to the SARS-CoV-2 spike protein receptor-binding domain was tested post-V2 (Roche Elecsys®). As recommended by the manufacturer, titers ≥0.8 U/mL were considered positive, the highest value being >250 U/mL. Blood samples were also collected before V1 and at distance from V2 to evaluate, by flow cytometry, total lymphocyte (Ly) counts and quantitative Ly subsets (CD3, CD4 and CD8 T cells, B and NK cells). Statistical analyses were performed on R (version 4.0.3). Patient characteristics were compared by using the Χ² test for discrete variables and the Wilcoxon test for continuous variables. Generalized linear models were used to conduct multivariate analyses. Results Samples were available from 117 Allo-HSCT patients who had been vaccinated between January 20 and April 17, 2021. Patient characteristics are provided in Table 1. The average interval from Allo-HSCT day 0 (D0) to V1 (D0-V1) was 654 (IQR: 372-1367) days (d). S-antibody response rate post-V2 was 82.9% for the entire cohort. Non-humoral responders (NHR) post-V2 (n = 20) had a lower D0-V1 interval (median 271 vs 914 d, p <10 -5) and a lower pre-V1 median total Ly count (0.62 vs 1.61x10 9/L, p < 10 -4). Lymphocyte subsets possibly predictive of antibody response were then investigated. NHR were associated with lower median CD3 (0.39 vs 0.97 x10 9/L, p = 0.01), CD4 (0.13 vs 0.35 x10 9/L, p=<10 -3), and B-cell (0.00 vs 0.28 G/L, p <10 -6) counts. NK and T CD8 counts were not statistically different between NHR and HR (respectively p=0.14 and p=0.06). No influence either was observed when considering the age of donors (p=0.39) or recipients (p=0.55), underlying disease (p=1), Allo-HSCT conditioning (p=0.11), blood groups (donor, p=0.55; receiver, p=0.39) or a previous history of graft versus host disease (GVHD; 83.1 vs 83.6%, p=1). Conversely, ongoing immunosuppressive (IS)/chemotherapy treatment and a haploidentical source of graft were associated with lower responses to vaccination (respectively 62.5 vs 90.5%, p<10 -3, and 69.4 vs 88.6% for patients with matched donors, p=0.02). In multivariate analysis (Fig.1) also including D0-V1 interval, donor source, current IS/chemotherapy treatment and TCD4 Ly count, only B cell aplasia remained statistically associated with lack of antibody response after two vaccine injections (OR 0.01, 95%CI [0.00 - 0.10], p <10 -3). The possible modification in terms of lymphocyte counts between pre-V1 and post-V2 times has been also investigated showing that only CD4 lymphocytes counts improved significantly (0.31 vs 0.34 x10 9/L, p=0.01) between this interval. Conclusion B cell aplasia appears as a major predictor of anti SARS-CoV-2 mRNA vaccine failure after Allo-HSCT. It may be suggested from this result that a close immune monitoring should be proposed after allotransplant to propose the vaccine at the most appropriate time, meaning after of B cell detection, regardless of the delay from Allo-SCT or the presence of an IS/chemotherapy treatment. The possibility for these patients to have mounted a cellular response should also be considered, which was not investigated here. Figure 1 Figure 1. Disclosures Moreau: Celgene BMS: Honoraria; Sanofi: Honoraria; Abbvie: Honoraria; Oncopeptides: Honoraria; Amgen: Honoraria; Janssen: Honoraria.

The efficacy of two doses of anti-severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) messenger RNA (mRNA) vaccines, successfully demonstrated in healthy populations, is progressively reported in recipients of allogeneic haematopoietic stem cell transplant (allo-HSCT) with, surprisingly, high humoral antibody responses of ~80%. In our own experience based on an observational study of 117 allo-HSCT adult recipients, we found that 83% of them achieved a significant specific humoral response after two doses (V1 and V2) of BNT162b2 anti-SARS-CoV-2 mRNA vaccine (Pfizer BioNTech). However, although 61 5% of the patients achieved the highest detectable antibody level, this proportion remained significantly lower than what was observed in healthy controls, where 100% reached this result. In the present study, we investigated retrospectively whether a third dose of BNT162b2 vaccine (V3) would improve the anti-SARS-CoV-2 response in a cohort including most of our previously reported patients. Patients with clinical or asymptomatic biological coronavirus disease 2019 (COVID-19) infection before V1 were excluded from the study. A cohort of healthy volunteers (caregivers from the Clinical Hematology Department) who had also already received V1 and V2 was considered as controls. All participants were vaccinated between January 20 and June 1, 2021. The study was approved by the Ethic Review Board of Nantes University Hospital. Antibody response to the SARS-CoV-2 Spike protein receptor-binding domain was tested after V2 for all subjects (Serology post V2, SpV2). All subjects benefited later from another evaluation of specific serum antibodies as monitoring (Serology post V2+, SpV2+) or after V3 (Serology post V3, SpV3). Antibody levels to the SARS-CoV-2 Spike protein receptor-binding domain were assayed in all participants using Roche Elecsys (Rotkreuz, Switzerland) with titres ≥0 8 u/ml considered positive, the highest threshold being >250 u/ml. Four subgroups could be identified: (i) negativity at both SpV2 and SpV2+/SpV3, (ii) increase of antibody levels between SpV2 and SpV2+/SpV3, including patients showing seroconversion, (iii) decreased or stable antibody levels between SpV2 and SpV2+/SpV3 and (iv) sustained highest antibody levels at both times. A cohort of 25 controls and 102 patients, including 80 who received V3 (V3+) and 22 who did not (V3 ) was considered for the purpose of the present study. The characteristics of participants and delays from SpV2 to SpV2+ or SpV2 to SpV3 are reported in Tables I and II. The V3+ and V3 subgroups shared similar characteristics except regarding the number of matched unrelated donors, which was significantly higher for the former (Table I). The reasons for not receiving V3 were forgetting or refusal. Moreover, although the French National Authority of Health had recommended the use of a third dose in immunocompromised hosts, surveillance after detection of the highest antibody level at SpV2 was retained for some patients. Samples from controls showed that they reached and sustained the highest anti-Spike antibody values (>250 u/ml) at both SpV2 and SpV2+, suggesting a persistent response without the need of a third vaccine in this healthy population. The proportion of allo-HSCT patients still negative at SpV2+ or SpV3 was not significantly different between V3 and V3+ patients (23% vs. 11%, P = 0 30). An increase in antibody levels between SpV2 and SpV3 was observed in 19 V3+ patients (24%), while none of V3 patients displayed higher levels than at SpV2 (P = 0 02) Among V3 patients, seroconversion occurred for two and 17 reached the highest antibody level, confirming the interest of a third booster. This led to 81% of V3+ patients with the highest antibody level versus 50% in the V3 group (P = 0 006), of note still significantly lower than in controls (P = 0 04). Only 5% of V3+ patients had stable/decreased antibody levels versus 27% of V3 subjects (P = 0 006) (Table II). Finally, the proportion of patients documented twice with the highest antibody level at SpV2 and SpV2+/SpV3 was similar between V3 and V3+ patients (50% vs. 60%, P = 0 55). Of note, neither COVID-19 infection nor graft-versus-host disease (GVHD) reactivation were documented in any participant. As already reported in solid organ transplant recipients, and in a recent report with allo-SCT recipients, this retrospective study demonstrates that the administration of a third vaccine dose to allotransplanted patients increases the humoral response and antibody levels. One major limitation of the present retrospective study is the absence of antibody level values after serum dilution (not done routinely in our hospital) and assays for neutralising antibodies. This would have been of high interest as higher titres seems to correlate with higher vaccine efficacy, while virus neutralising response towards variants may depend on the number of vaccines and neutralising antibody titres. correspondence

This was a monocentric prospective study testing the efficacy and safety of a first injection of BNT162b2 (Pfizer-BioNTech) in 112 Allo-HSCT patients. Antibody response to SARS-CoV-2 spike protein receptor-binding domain was tested at the time of the second injection (Roche Elecsys). The study also included a non-randomized control arm of 26 healthy controls. This study shows that a first dose of SARS-CoV-2 messenger RNA vaccine is safe and provides a 55% rate of seroconversion in allotransplanted patients compared to 100% for the controls (p < 0.001). Factors influencing the absence of response in patients were recent transplantation (<2 years), lymphopenia (<1 × 109/L) and immunosuppressive treatment or chemotherapy at the time of vaccination.