The availability of long-acting mAbs for preexposure prophylaxis presented an additional option for solid organ transplant recipients who have been vaccinated but may still be at inordinate risk for COVID-19, and mAbs are an additional consideration for recipients of SOT who have not yet been vaccinated and who may have multiple risk factors associated with a lower immune response to vaccination. Notes Supplement.eMethods. vaccine doses. Older age, recent transplantation, deceased donor status, active use of antimetabolites, and recent exposure to antithymocyte globulin or rituximab were risk factors associated with diminished humoral immune response after receiving 2 doses of mRNA vaccines. Meaning These findings suggest that more efforts are needed to modulate the risk factors associated with reduced humoral responses among recipients of solid organ transplant. Abstract Importance Recipients of solid organ transplant (SOT) Tnfsf10 experience decreased immunogenicity after COVID-19 vaccination. Objective To summarize current evidence on vaccine responses and identify risk factors for diminished humoral immune response in recipients of SOT. Data Sources A literature search was conducted from existence of database through December 15, 2021, using MEDLINE, Embase, Web of Science, Cochrane Library, and ClinicalTrials.gov. Study Selection Studies reporting humoral immune response of the COVID-19 vaccines in recipients of SOT were reviewed. Data Extraction and Synthesis Two reviewers independently extracted data from each eligible study. Descriptive statistics and a random-effects model were used. This report was prepared following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline. Data were analyzed from December 2021 to February 2022. Main Outcomes and Measures The total numbers of positive immune responses and percentage across each vaccine platform were recorded. Pooled odds ratios (pORs) with 95% CIs were used to calculate the pooled effect estimates of risk factors for poor antibody response. Results A total of Chlorhexidine 83 studies were included for the systematic review, and 29 studies were included in the meta-analysis, representing 11?713 recipients of SOT. The weighted mean (range) of total positive humoral response for antispike antibodies after receipt of mRNA COVID-19 vaccine was 10.4% (0%-37.9%) for 1 dose, 44.9% (0%-79.1%) for 2 doses, and 63.1% (49.1%-69.1%) for 3 doses. In 2 studies, 50% of recipients of SOT with no or minimal antibody response after 3 doses of mRNA COVID-19 vaccine mounted an antibody response after a fourth dose. Among the factors associated with poor antibody response were older age (mean [SE] age difference between responders and nonresponders, 3.94 [1.1] years), deceased donor status (pOR, 0.66 [95% CI, 0.53-0.83]; value of Egger regression was values were 2-sided, and statistical significance was set at em P /em ?=?.05. Data were analyzed from December 2021 to February 2022. Results Study and Patient Characteristics Our initial search generated 2832 studies; 896 studies were removed because they were duplicates, and 1748 studies were excluded by screening through the titles and abstracts. We performed full-study reviews on 188 articles. After review, 105 articles were excluded owing to being a review article, case report, preprint, incorrect patient population, or duplicate cohort or having no outcomes of interest. A total of 83 studies9,10,11,12,13,14,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106 were included in the systematic review, of which 29 studies were included in the meta-analysis (Figure 1). The characteristics of 83 included studies are described in eTable 2 in the Supplement. There were 11?713 study participants across all studies, including heart, lung, heart-lung, liver, kidney, pancreas, kidney-pancreas, and other combined transplantation. Grading of recommendation assessment, development and evaluation for potential factors associated with seroconversion was reported in Chlorhexidine eTable 3 in the Supplement.107 Open in a separate window Figure 1. Study Selection Flowchart Humoral Immune Responses mRNA Vaccines A total of 83 studies of immunogenicity of the mRNA COVID-19 vaccines in study participants were identified. Of these, 18 studies reported antibody response after 1 dose, 54 studies after 2 doses, 11 studies after 3 doses, and 2 studies after 4 doses of the mRNA COVID-19 vaccines. Among Chlorhexidine the studies analyzed, the weighted mean (range) seroconversion rate after 1 dose of mRNA vaccine was 10.4% (0%-37.9%) for antispike antibodies.