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Background: High titer COVID-19 convalescent plasma (CCP) reduces hospitalizations among immunocompetent outpatients. This study evaluated recipient post-transfusion S receptor binding domain (S-RBD) IgG antibody levels and the association of progressing to hospitalization among unvaccinated outpatients with COVID-19 treated with CCP or control plasma. Method(s): This analysis focused on participants from a multicenter doubleblind, randomized, controlled trial comparing treatment of outpatients with COVID-19 convalescent plasma (CCP) or control plasma without SARS-CoV-2 antibodies. Participants with confirmed SARS-CoV-2 infection were transfused within 9-days of symptom onset between June 2020 and October 2021 (n=110 vaccinated control;n=105 vaccinated CCP;n=464 unvaccinated control;n=472 unvaccinated CCP;total n=574 control and n=577 CCP recipients). All subjects had specimens collected the day prior to transfusion (D-1), within 30 minutes after transfusion (D0), 14 (D14), 28 (D28), and 90 (D90) days post-transfusion. Ancestral SARS-CoV-2 S-RBD was measured by an in-house validated ELISA. All 54 COVID-19-related hospitalizations occurred within 2 weeks of transfusion. Result(s): Post-transfusion anti-S-RBD IgG levels on D0 were significantly greater for CCP (median=4 titer,log3) compared to control (median=2 titer,log3;p< 0.001) recipients. Neither sex nor age impacted antibody levels following CCP treatment at D14, D28, and D90. Vaccinated recipients had greater titers than unvaccinated recipients prior to transfusion with little change in titers post-transfusion. Unvaccinated recipients had low antibody titers on D-1 with CCP recipients exhibiting a significant increase in titer from D-1 to D0 compared to controls (mean fold change=1.89;p< 0.001). Among unvaccinated recipients, those who received CCP transfusion late ( >5 days after symptom onset) and had low D0 antibody levels (< 4.24 titer, log3) had the greatest proportion of hospitalizations (5.5%). In contrast, those who received CCP transfusion early (< 5 days after symptom onset) with high D0 antibody levels ( >4.24 titer, log3) had no hospitalizations. Unvaccinated CCP recipient anti-S-RBD IgG antibody levels on D0 correlated with donor anti-S-RBD IgG antibody levels (r=0.30, p< 0.001). Conclusion(s): Among unvaccinated outpatients with COVID-19, CCP recipient antibody dilutional titers after transfusion over 540 titer correlated with protection against hospitalization when transfusion occurred within 5 days of symptom onset. (Figure Presented).
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Background: Sero-studies of SARS-CoV-2 have used antibody (Ab) responses to spike (S) and nucleocapsid (N) antigens to differentiate mRNA vaccinated (S+/N-) from infected (S+/N+) individuals. We performed testing on wellcharacterized subjects to determine how repeated vaccination or infection, and time from those exposures, influence these Ab levels. Method(s): Samples from individuals with known infection status: prepandemic negative controls n=462;first-time infected n=237 (~45 days post);vaccinated after infection n= 34 (~40 days post-vaccination and ~180 days post-infection);fully vaccinated n=158 (~50 days post);boosted n=31 (~30 days post);breakthrough n=18 (~14 days post-infection);reinfected n=10 (varied). Longitudinal samples (n=51) from subjects with evidence of reinfection (symptoms and/or positive rapid antigen test), were tested to determine the impact of the order of infection and/or vaccination on the magnitude of the anti-S and anti-N IgG Ab detected in the blood. Testing was performed with MesoScale Diagnostics (Gaithersburg, MD) assay. Outcomes are presented in WHO International Binding Antibody Units (BAU/mL). The cutoff for a positive result was 18 BAU for S and 12 BAU for N. Result(s): The median amount of Ab (IQR) in BAU for each group (Figure A) was: pre-pandemic negative controls S 0.53(0.27,1.03), N 0.55(0.18,1.67);first-time infected S 114(51,328), N 70(29,229);vaccinated after infection S 4367(2479,4837), N 15(7,35);fully vaccinated S 998(586,1529), N 0.31(0.16,0.68);boosted S 2988(1768,3522), N 0.59(0.32,1.03);breakthrough S 2429(2032,3413), N 2.5(0.93,8.6);reinfected S 1533(486,4643), N 7.8(2.6,62). For the breakthrough and second infections 17% and 40% were seropositive to N, respectively. Longitudinal analysis (Figure B) of those with multiple infections showed that all those with a positive rapid antigen test for their second infection had an increase in N Ab. Conclusion(s): The prevalence of antibodies to nucleocapsid cannot be used to determine the proportion of individuals infected to SARS-CoV-2 in a vaccinated population. Booster, repeated, and breakthrough infections are associated with IgG Ab levels to S >400 BAU/mL. A majority of breakthrough infections did not elicit an Ab response to N. For those with repeated infection, a minority elicited antibody responses to N. This could be related to misdiagnosis or the burden of infection, as only those who were positive by rapid antigen assay (indicative of a high viral load) had an increase in N Ab.
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Background: COVID-19 in Africa was less severe with fewer reported cases, hospitalizations and deaths compared to other continents. However, the lack of adequate surveillance systems in Africa makes estimating the burden of infection challenging. Serosurveillance can aid in determining the frequency of infection within this population. This study is aimed to estimate SARS-CoV-2 seroprevalence, describe the SARS-CoV-2 antibody (Ab) levels, and examine associations of seroreactivity among Ugandan blood donors. Method(s): Samples were obtained from the Mirasol Evaluation of Reduction in Infections Trial (MERIT), a randomized, double blind, controlled clinical trial evaluating transfusion transmitted infections. MERIT blood donor samples (n=3,517) were collected from Kampala, Uganda between October 2019 to April 2022. Additional blood donor samples (n=1,876) were collected from around the country between November-December 2021. Samples were tested for Ab to SARS-CoV-2 nucleocapsid (N) and spike (S) using an electrochemiluminescence immunoassay assay (Meso Scale Diagnostics, Gaithersburg, MD) per manufacturer's protocol. Samples seroreactive to both N and S Ab were considered Ab positive to SARS-CoV-2. Seroprevalence among MERIT donors were estimated within each quarter. Factors associated with seroreactivity from November-December 2021 were assessed by chi-square test. Result(s): SARS-CoV-2 seroprevalence increased from < 2.0% in October 2019- June 2020 to 82.5% in January-April 2022. Three distinct peaks in seroreactivity were seen in October-November 2020, July-August 2021, and January-April 2022 (see Figure). Among seroreactive donors, median N Ab levels increased 9-fold and median S Ab 19-fold over the study period. In November-December 2021, SARS-CoV-2 seroprevalence was higher among donors from Kampala (58.8%) compared to more rural regions of Hoima (47.7%), Jinja (47.9%), and Masaka (54.4%;p=0.007);S seroprevalence was lower among HIV+ donors (58.8% vs. 84.9%;p=0.009). Conclusion(s): Blood donors in Uganda showed high prevalence of Ab to SARSCoV- 2 by March of 2022, indicating that the infection levels were similar to many other regions of the globe. Higher seroprevalence was observed in the capital compared to more rural areas in Uganda. Further, increasingly high antibody levels among seropositive donors may indicate repeat infections. The lower COVID-19 morbidity and mortality was not due to a lack of exposure of the virus, but other factors yet to be determined.
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Background/Case Studies: Twitter Social Media has grown in traction as a freely available worldwide platform for the real-time rapid dissemination of medical knowledge, a means of fostering engagement and building communities of practice. Twitter Space is a new live audio platform on Twitter. Like tweets, spaces are public and can be accessed by anyone, thus serving as another way to break traditional boundaries of accessing knowledge. In January 2022, Transfusion News developed a program on Twitter Space called #Blooducation Baristas with the goal of providing a venue for a small panel of experts to discuss an interesting and current topic. Here we share our early experiences using this innovative tool to advance transfusion medicine education. Study Design/Methods: Episodes are scheduled every eight weeks and are 30-min discussions using a questionand- answer format on a burgeoning transfusion medicine topic. Once the topic is designated, an investigation on the leading experts is considered and invited to partake in a panel discussion. Of those who do not have a Twitter account, detailed instructions are provided as well as virtual tutorials on navigating the platform. Once confirmed, a set of preformed questions are provided to the expert panelists. The episodes are advertised through the Transfusion News website, emails and Twitter posts from the moderators and speakers one to two weeks in advance of the sessions. Using email analytics, Tweepsmap and Twitter Analytics, engagement was captured on the following items: number of Twitter Space listeners, number of Tweet impressions, number of new Twitter followers, geographic location of Twitter engagement, and number of new email subscribers. Results/Findings: Over a three-month period, two #Blooducation Barista episodes have been aired on Twitter Space. The first episode took place on January 28, 2022 and covered the topic of COVID-19 convalescent plasma in the Omicron era. The second episode took place on March 25, 2022 and covered the topic of Paid Platelet Donors. Over three months, over 200 listeners attended the sessions from over three countries. The second episode generated three new email subscribers across three including United States, Canada and Qatar. Feedback from panelists included that the experience was "fun and easy" and well as a great opportunity to interact with other experts and discuss different points of view. Conclusion(s): The use of different tools offered by social media is a suitable and efficient strategy to bring experts together and reach out to a broad, international, and diverse audience about relevant and current topics. Additionally it provides another avenue for interactive engagement and reaching new subscribers by using a different platform to attract learners.
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Purpose: Kidney transplant recipients (KTRs) have diminished immune response and protection after 2-dose mRNA COVID-19 vaccination. It is unknown if additional doses improve neutralization of variants of concern (VOC) in KTRs with prior poor seroresponse. Method(s): Adult KTRs with negative (<0.8 U/mL) or low (<=50 U/ml) anti-RBD Ig (Roche Elecsys anti-SARS-CoV-2-S) after 2-dose mRNA series were given a homologous 3rd dose (D3). Anti-RBD and VOC surrogate neutralization (%ACE2i) were measured 30 days post D3;responses were stratified by baseline anti-RBD. Reactogenicity, serial SARS-CoV-2 swabs, and donor-specific antibody (DSA) were assessed. Result(s): 81 KTRs (50% negative anti-RBD) received D3 (72% BNT162b2, 28% mRNA-1273) at median 167 days post D2 (Table). Median (IQR) anti-RBD increase was 410 (8-2309) U/mL with 69% (40% negative vs 98% low anti-RBD) achieving day 30 anti-RBD >50 U/ml (Fig1a). 22% remained seronegative. Non-response was associated with lower baseline lymphocyte count (median 770 vs 1160 cells/ uL;p=0.05) and IgG (median 779 vs 979 mg/dL;p<0.01), but not demographics, vaccine, or immunosuppressives. Median (IQR) delta variant %ACE2i increased from 6% (3-7) to 10% (4-22) (p<0.001), a 1% (0-5) increase in negative vs 13% (5-25) in low anti-RBD. %ACE2i was linearly associated with anti-RBD >=100 U/ mL (all VOC shown in Fig1b);64% of KTRs with anti-RBD >=250 U/mL had delta %ACE2i >20. There were 3 cases of mild-moderate COVID-19 >=7 days post-D3, with pre-infection anti-RBD <0.4, 22, 76 U/mL and delta %ACE2i 6, 9, and 16, respectively. There was no acute rejection, nor increased or de novo DSA. Conclusion(s): A 3rd mRNA vaccine dose increased anti-RBD and VOC neutralization in KTRs without inducing clinical alloimmunity, yet 45% with negative baseline anti-RBD remained seronegative without delta variant neutralization. Trials are ongoing to test immune response augmentation in this subgroup via temporary immunosuppression reduction or heterologous boosting.
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Purpose: Liver transplant (LT) recipients have a decreased response to 2 doses of SARS-CoV-2 vaccine compared to the general population, so we aimed to understand response to a third dose to inform vaccination strategies. Method(s): LT recipients in our observational cohort who received 3 homologous mRNA vaccines and available antibody levels pre- and post-dose 3 (D3) were identified. Those who reported a prior COVID-19 diagnosis or used belatacept were excluded. The peak anti-spike antibody level collected between the second (D2) and third dose (D3), was compared to the antibody level at 1 month post-D3. Samples were tested with Roche Elecsys Anti-Sars-CoV-2 enzyme immunoassay (EIA) (positive >=0.8 U/mL) or EUROIMMUN EIA (positive >=1.1 AU). Result(s): 146 participants completed 3 homologous doses of BNT162b2 (53%) or mRNA-1273 (47%) vaccines between 5/15/2021 - 11/8/2021. The median (IQR) time of peak pre-D3 antibody collection was 89 (31, 104) days post-D2. The median time of 1-month post-D3 antibody collection was 30 (23, 33) days. The median time between D2 and D3 was 168 (149-188) days. Overall, 125/146 (86%) were seropositive pre-D3, and 139/146 (95%) were seropositive post-D3 (Figure 1). There were no seroreversions post D3, and among the 21 seronegative recipients pre-D3, 14 (67%) seroconverted post-D3. Risk factors significantly associated with persistent seronegativity post-D3 were less time since LT (1.3 vs 6 years, p=0.042), mycophenolate use (100% vs 37%, p=0.001), BNT162b2 series (100% vs 50%, p=0.01), and pre-D3 seronegative status (86% vs 10%, p<0.001). Conclusion(s): Most LT recipients have excellent responses to a third homologous mRNA vaccine dose, greater than that seen in other transplant recipients. Persons seronegative after D2, however, show weaker response and may remain at high risk for SARS-CoV-2 infection despite D3.
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Purpose: Kidney transplant recipients taking belatacept (KTR-B) have poor immune response to two-dose SARS-CoV-2 vaccination. We sought to characterize the impact of an additional vaccine dose on plasma neutralizing capacity and cellular responses as compared to that of KTRs controls (KTR-C) not taking belatacept. Method(s): Within an observational cohort, we tested 26 KTR-Bs and 27 KTR-Cs for anti-spike antibody responses before and after a third SARS-CoV-2 vaccine dose (D3) using two clinical assays (Roche Elecsys anti-S Ig and EUROIMMUN anti-S1 IgG). For a subset of 5 KTR-Bs and for all KTR-Cs we used a research assay (Meso Scale Diagnostics V-Plex [MSD]) to further assess anti-spike and RBD IgG, as well as surrogate plasma neutralizing activity (% ACE2 inhibition) versus the ancestral and delta variants. For 3 KTR-Bs, post D3 T cell response was assessed via IFN-y ELISpot and deemed positive if spot forming units > 20 per million PBMC and stimulation index > 3. Result(s): KTR-Bs had significant lower clinical anti-spike seroconversion than KTR-Cs (31% vs 74%, p=0.001) after D3 despite similar demographics, clinical factors, and vaccines administered (Table 1). No KTR-B (0/5) was seropositive by MSD anti-spike or anti-RBD IgG (Figure 1). % ACE2 inhibition versus the ancestral variant was significantly lower in KTR-Bs than in KTR-Cs (Median [IQR] 5.2 [2.8, 6.5] vs 12.5 [7.7, 23.9], p<0.01);all KTR-Bs were below a level consistent with detectable neutralizing antibody. All tested KTR-Bs (3/3) had a negative ELISpot, consistent with negligible cellular response. Conclusion(s): These results suggest minimal humoral or cellular immunogenicity of additional vaccine doses for KTR-Bs and indicates the need for alternative strategies to improve vaccine response such as immunosuppression alteration or use of passive immunoprophylaxis with monoclonal anti-spike antibody to improve protection versus SARS-CoV-2.
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Purpose: Solid organ transplant recipients (SOTRs) are at increased risk for severe COVID-19 and exhibit lower antibody responses to SARS-CoV-2 vaccines. This study aimed to determine if pre-vaccination cytokine levels are associated with antibody response to SARS-CoV-2 vaccination. Method(s): A cross-sectional study was performed among 58 SOTRs before and after two-dose mRNA vaccine series, 35 additional SOTRs before and after a third vaccine dose, with comparison to 16 healthy controls (HCs). Anti-spike antibody was assessed using the IgG Euroimmun ELISA. Electrochemiluminescence detectionbased multiplexed sandwich immunoassays were used to quantify plasma cytokine and chemokine concentrations (n=20 analytes). Concentrations between SOTRs and HCs, stratified by ultimate antibody response to the vaccine, were compared using Wilcoxon-rank-sum test with false discovery rates (FDR) computed to correct for multiple comparisons. Result(s): In the study population, 100% of HCs, 59% of SOTRs after two doses and 63% of SOTRs after three doses had a detectable antibody response. Multiple baseline cytokines were elevated in SOTRs versus HCs. There was no significant difference in cytokine levels between SOTRs with high vs low-titer antibodies after two doses of vaccine. However, as compared to poor antibody responders, SOTRs who went on to develop a high-titer antibody response to a third dose of vaccine had significantly higher pre-third dose levels of several innate immune cytokines including IL-17, IL-2Ra, IL-6, IP-10, MIP-1alpha, and TNF-alpha (FDR <0.05). Conclusion(s): A specific inflammatory profile or immune state may identify which SOTRs are likely to develop stronger sero-response and possible protection after a third dose of SARS-CoV-2 vaccine.
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Purpose: The impact of antigenic imprinting, when immune memory of one antigen influences the response to subsequent similar antigens, on the antibody response in solid organ transplant recipients (SOTRs) after SARS-CoV-2 vaccination is currently unknown. This study examines the relationship between seasonal coronaviruses (sCoV) and SARS-CoV-2 antibody levels pre- and post-vaccination in SOTRs. Method(s): Plasma from 52 SOTRs pre- and post-SARS-CoV-2 vaccination (2 doses, mRNA) was analyzed using the Meso Scale Diagnostic Coronavirus Panel 3 (an electrochemiluminescence detection-based multiplexed sandwich immunoassay) for IgG antibodies against alpha sCoVs (229E, NL63), beta sCoVs (HKU1, OC43), and SARS-CoV-2 spike proteins. Changes in IgG titers were determined by paired Wilcoxon rank-sum tests. Spearman correlation analysis was used to determine associations between pre-vaccination anti-sCoVs and post-vaccination anti-SARS-CoV-2 IgG. Result(s): Vaccination increased both anti-SARS-CoV-2 (fold change (FC) 1.9, p<0.001) and anti-beta sCoV (HKU1 [FC 0.05, p<0.001], OC43 [FC 0.8, p<0.001]) IgG titers in SOTRs, but did not increase anti-alpha sCoV IgG. Furthermore, prevaccination anti-beta sCoV (HKU1 [rho= -0.3, p=0.03], OC43 [rho= -0.3, p<0.03]) IgG titers were negatively correlated with post-vaccination anti-SARS-CoV-2 IgG. Conclusion(s): These exploratory findings suggest that prior exposure to seasonal betacoronaviruses may lead to antigenic imprinting in SOTRs that negatively impacts the antibody response to vaccination against the novel pandemic betacoronavirus, SARS-CoV-2.
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Purpose: Humoral response to COVID-19 vaccines is attenuated in many solid organ transplant recipients (SOTRs), necessitating additional primary and booster vaccinations. The omicron variant demonstrates substantial immune evasion, and it is not known if boosters increase neutralizing capacity versus omicron among SOTRs. We therefore investigated SOTR antibody response and neutralization versus variants of concern (VOC) including omicron to a 4th vaccine dose (D4). Method(s): Within a national, prospective observational cohort, 25 SOTRs underwent anti-SARS-CoV-2 spike and receptor binding domain (RBD) IgG testing using the Meso Scale Discovery platform before and 2-4 weeks after D4. Surrogate neutralization (%ACE2 inhibition [%ACE2i], range 0-100% with >20% correlating with live virus neutralization), was measured versus full spike proteins of the ancestral ("vaccine") strain and 5 VOCs including delta and omicron. Change in IgG level and %ACE2i were compared using paired Wilcoxon rank-sum testing. Result(s): Demographics are outlined in Table 1, including median (IQR) age 59 (45- 55) years, 64% kidney recipients, and D4 receipt (60% Moderna, 40% Pfizer) median (IQR) 93 days (28-134) post D3. Two participants had SARS-CoV-2 exposure per anti-nucleocapsid testing, including one incident infection. Overall, anti-RBD (92%- >100%) and anti-spike (84%->92%) seropositivity increased after D4, as did median (IQR) anti-spike IgG 42.3 (4.9-134.2)->228.9 (115.4-655.8) WHO binding antibody units (p<0.05). Median (IQR) %ACE2i significantly increased after D4 vs the vaccine strain 5.8% (0-16.8)->20.6% (5.8-45.9) and delta variant 9.1% (4.9-12.8)->17.1% (10.3-31.7) (both p<0.001). In contrast, no SOTR showed neutralization vs omicron before or after D4: median (IQR) %ACE2i 4.1% (0-6.9)->0.5% (0-5.7) (p=0.11). Conclusion(s): Although a 4th vaccine dose increased anti-spike IgG and neutralizing capacity vs some VOC, there was no omicron variant neutralization among SOTRs. SOTRs may remain at high risk for SARS-CoV-2 infection despite boosting, thus additional protective interventions should be urgently explored. (Figure Presented).
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Purpose: nti-spike antibody response to SARS-CoV-2 vaccination is diminished in LT recipients compared to the general population so understanding durability for those that do respond is critical to mitigating risks of infection. We measured serial antibody titers in LT recipients for 6 months after two-dose mRNA vaccine series to describe kinetics and sero-reversion rates. Method(s): LT recipients without known prior COVID-19 had anti-spike antibody testing at 1, 3, and 6 months after the second dose of mRNA vaccine (D2) using two commercial assays (Roche Elecsys anti-receptor binding domain immunoassay (EIA) [positive >=0.8 U/mL] or EUROIMMUN anti-S1 EIA [positive >=1.1 AU]). We compared titer distributions over time and identified factors associated with sero-reversion. Result(s): 180 LT recipients received BNT162b2 (48%) or mRNA-1273 (52%) 2-dose series between 1/7/2021-5/7/2021. At 1 month post-D2 (n=173), 146 (84%) had positive antibody levels at a median (IQR) of 30 (28, 32) days post-D2. At 3 months post-D2 (n=164), 149 (91%) had positive levels at a median of 92 (90, 96) days post-D2. At 6 months post-D2 (n=73), 62 (85%) had positive levels at a median of 180 (176, 185) days post-D2. Among the 66 seropositive at 1 or 3 months post-D2, 58 (88%) remained seropositive by 6 months post-D2. Neither age, years since transplant, vaccine type, nor mycophenolate (MMF) use were associated with sero-reversion, though there was a trend toward more triple immunosuppressive use (25% vs 3%, p=0.07). Of those Roche-tested, the median anti-RBD levels were >=250 U/mL (14, >=250;n=120) at 1 month post-D2, >=250 U/mL (58, >=250;n=113) at 3 months, and >=250 U/mL (30, >=250;n=49) at 6 months . Of those EUROIMMUN-tested, the median anti-S1 levels were 7.25 AU (4.31, 8.71;n=53) at 1 month, 5.71 AU (1.27, 7.90;n=51) at 3 months, and 1.73 AU (0.76, 6.01;n=25) at 6 months. Conclusion(s): Overall, most LT recipients demonstrated 6 month durability of anti-spike antibody following vaccination, but a subset did sero-revert, potentially associated with heavier immunosuppression. Further investigation into clinical consequences of waning antibody levels is key to guide timing of additional vaccine doses.
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Purpose: Heart and lung transplant (HT/LT) recipients have impaired humoral responses to SARS-CoV-2 vaccination compared to other solid organ transplant recipients (SOTRs). The purpose of this study is to describe antibody titer kinetics and durability among HT and LT recipients. Method(s): HT or LT recipients (> 18 years) with no known COVID-19 infection were included. Demographics and clinical characteristics were collected via survey. Serologic testing was performed on the Roche Elecsys anti-SARS-CoV-2 enzyme immuno-assay (EIA) or the EUROIMMUN EIA pre- and post-dose 2 (D2). Result(s): Among 93 HT recipients, 59 (63%) were seropositive 1 month and 66 (71%) 3 months post-D2 (Table 1). Seropositive HT recipients had a higher median length of time from transplant to vaccination. 7/66 (11%) had delayed seroconversion (were negative for antibodies 1-month post-D2). Median(IQR) anti-RBD was 81 (8, 250) 1-month post-D2 (n=38) and 231 (48, 438) U/mL 3-months post-D2 (n=43) (Figure 1). Among 68 LT recipients, 29/68 (43%) were seropositive 1-month and 30 (44%) 3-months post-D2. Seronegative LT recipients were more likely to younger (18-39 years old, 15% vs 3%), or older (> 60 years, 74% vs. 50%, p=0.01). Seronegative LT recipients were more likely to be on anti-metabolite therapy (79% vs. 53%, p=0.04) and had a lower median length of time from transplant to vaccination. Among seropositive LT recipients at 3-months, 3 (10%) had delayed seroconversion. Median (IQR) anti-RBD was 61 (4, 233) U/mL 1-month post-D2 (n=26) and 45(11, 299) U/mL 3-months post-D2. Conclusion(s): HT and LT recipients develop a delayed and variable antibody response to mRNA SARS-CoV-2 vaccination. HT recipients more frequently seroconverted, and had higher anti-RBD levels, than LT recipients. Persistent negative and low antibody titers may place LT recipients at the highest risk of breakthrough SARSCoV- 2 infection among SOTRs.