Pure laparoscopic living donor liver transplantation: Dreams come true.

Minimally invasive approaches are increasingly being applied in surgeries and have recently been used in living donor hepatectomy. We have developed a safe and reproducible method for minimally invasive living donor liver transplantation, which consists of pure laparoscopic explant hepatectomy and pure laparoscopic implantation of the graft, which was inserted through a suprapubic incision. Pure laparoscopic explant hepatectomy without liver fragmentation was performed in a 60-year-old man with alcoholic liver cirrhosis and hepatocellular carcinoma. The explanted liver was retrieved through a suprapubic incision. A modified right liver graft, procured from his 24-year-old son using the pure laparoscopic method, was inserted through a suprapubic incision, and implantation was performed intracorporeally throughout the procedure. The time required to remove the liver was 369 min, and the total operative time was 960 min. No complications occurred during or after the surgery. The patient recovered well, and his hospital stay was of 11 days. Pure laparoscopic living donor liver transplantation from explant hepatectomy to implantation was performed successfully. It is a feasible procedure when performed by a highly experienced surgeon and transplantation team. Further studies with larger sample sizes are needed to confirm its safety and feasibility.

Sequential hypothermic and normothermic machine perfusion enables safe transplantation of high-risk donor livers.

Ex situ normothermic machine perfusion (NMP) is increasingly used for viability assessment of high-risk donor livers, whereas dual hypothermic oxygenated machine perfusion (DHOPE) reduces ischemia-reperfusion injury. We aimed to resuscitate and test the viability of initially-discarded, high-risk donor livers using sequential DHOPE and NMP with two different oxygen carriers: an artificial hemoglobin-based oxygen carrier (HBOC) or red blood cells (RBC). In a prospective observational cohort study of 54 livers that underwent DHOPE-NMP, the first 18 procedures were performed with a HBOC-based perfusion solution and the subsequent 36 procedures were performed with an RBC-based perfusion solution for the NMP phase. All but one livers were derived from extended criteria donation after circulatory death donors, with a median donor risk index of 2.84 (IQR 2.52-3.11). After functional assessment during NMP, 34 livers (63% utilization), met the viability criteria and were transplanted. One-year graft and patient survival were 94% and 100%, respectively. Post-transplant cholangiopathy occurred in 1 patient (3%). There were no significant differences in utilization rate and post-transplant outcomes between the HBOC and RBC group. Ex situ machine perfusion using sequential DHOPE-NMP for resuscitation and viability assessment of high-risk donor livers results in excellent transplant outcomes, irrespective of the oxygen carrier used.

A novel microsurgical reconstruction technique in a living-related liver donor hepatic artery dissection using the native hepatic artery.

In living donor liver transplantation, hepatic artery intimal dissection is a rare but devastating complication often resulting in the inability to utilize the graft. We detail the salvage of a dissected donor right hepatic artery utilizing the recipient hepatic artery. After removal of the right lobe, the donor artery was found to have an intimal dissection extending to multiple branches. The liver transplant surgeons requested their plastic microsurgeon colleague to assist with reconstruction. Ultimately, the native recipient hepatic artery was used as a branch graft as the caliber and branching pattern was appropriate. Back table microvascular reconstruction was performed using the explanted recipient hepatic artery branches as a graft to the four donor artery branches. Every anastomosis was assessed with intraoperative doppler; all were patent with acceptable flow characteristics. The patient did well post-operatively with post-operative ultrasounds demonstrating patency of the graft. Four months post-transplantation the patient developed two polymicrobial abscesses that were drained and resolved with normalization of liver function tests. This case highlights how collaboration with a microvascular surgeon enabled the salvage of a living donor graft when faced with a complex arterial dissection.

Cross-decoration of dendritic cells by non-inherited maternal antigen-containing extracellular vesicles: Potential mechanism for PD-L1-based tolerance in cord blood and organ transplantation.

Exposure to non-inherited maternal antigens (NIMA) during the fetal period induces lifelong split tolerance to grafts expressing these allo-antigens. In adult mice, the production of extracellular vesicles (EVs) from maternal microchimeric cells causes cross-decoration (XD) of offspring dendritic cells (DC) with NIMA and upregulation of PD-L1, contributing to NIMA tolerance. To see how this may apply to humans, we tested NIMA acquisition by fetal DCS in human cord blood. The average percentage of NIMA-XD among total DCs was 2.6% for myeloid and 4.5% for Plasmacytoid DC. These cells showed higher PD-L1 expression than their non-XD counterparts (mDC: p = .0016; pDC: p = .024). We detected CD9+ EVs bearing NIMA and PD-L1 in cord blood. To determine if this immune regulatory mechanism persists beyond the pregnancy, we analyzed NIMA-expressing kidney and liver transplant recipients. We found donor antigen XD DCs in peripheral blood and graft-infiltrating DCs. As in cord blood, the pattern of donor antigen expression was punctate, and PD-L1 expression was upregulated, likely due to both protein and miRNA acquired from EV. Our findings support a mechanism for split tolerance to NIMAs that develops during pregnancy and is recapitulated in adult transplant recipients.

Alloantibodies after simultaneous liver-kidney transplant: A story of primary nonfunction, retransplantation, and antibody-mediated rejection.

Simultaneous liver-kidney transplant (SLKT) in the presence of antihuman leukocyte antigen (HLA) donor-specific antibodies (DSA) is a well-accepted practice. Herein, we describe the evolution of alloantibodies in a patient who received an SLKT. The pre-SLKT serum sample showed multiple strong DSA. As expected, all DSA cleared in a sample collected 4 days after the SLKT. Because of the primary nonfunction of the liver in the SLKT, the patient had a second liver transplant 4 days later. An abrupt increase in DSA levels against the kidney was detected 10 days after the second liver transplant. These DSA were refractory to treatment, and the transplanted kidney was lost due to antibody-mediated rejection (AMR). A detailed study of the HLA epitopes recognized by DSA and, after normalization with third-party alloantibodies to address the effect of multiple transfusions and liver allograft neutralization, showed that the elimination of these antibodies depended on the HLA antigens expressed by the transplanted liver cells. The return of DSA after removal of the first transplanted liver was associated with AMR in the transplanted kidney.

Outcome of liver transplantation with grafts from brain-dead donors treated with dual hypothermic oxygenated machine perfusion, with particular reference to elderly donors.

Prompted by the utilization of extended criteria donors, dual hypothermic oxygenated machine perfusion (D-HOPE) was introduced in liver transplantation to improve preservation. When donors after neurological determination of death (DBD) are used, D-HOPE effect on graft outcomes is unclear. To assess D-HOPE value in this setting and to identify ideal scenarios for its use, data on primary adult liver transplant recipients from January 2014 to April 2021 were analyzed using inverse probability of treatment weighting, comparing outcomes of D-HOPE-treated grafts (n = 121) with those preserved by static cold storage (n = 723). End-ischemic D-HOPE was systematically applied since November 2017 based on donor and recipient characteristics and transplant logistics. D-HOPE use was associated with a significant reduction of early allograft failure (OR: 0.24; 0.83; p = .024), grade ≥3 complications (OR: 0.57; p = .046), comprehensive complication index (-7.20 points; p = .003), and improved patient and graft survival. These results were confirmed in the subset of elderly donors (>75-year-old). Although D-HOPE did not reduce the incidence of biliary complications, its use was associated with a reduced severity of ischemic cholangiopathy. In conclusion, D-HOPE improves postoperative outcomes and reduces early allograft loss in extended criteria DBD grafts.

Successful living donor liver transplantation for liver failure due to maternal T cell engraftment following cord blood transplantation in X-linked severe combined immunodeficiency disease: Case report.

Maternal T cells from perinatal transplacental passage have been identified in up to 40% of patients with severe combined immunodeficiency (SCID). Although engrafted maternal T cells sometimes injure newborn tissue, liver failure due to maternal T cells has not been reported. We rescued a boy with X-linked SCID who developed liver failure due to engrafted maternal T cell invasion following living donor liver transplantation (LDLT) following unrelated umbilical cord blood transplantation (UCBT). After developing respiratory failure 3 weeks postpartum, he was diagnosed with X-linked SCID. Pathological findings showed maternal T cells engrafted in his liver and hepatic fibrosis gradually progressed. He underwent UCBT at 6 months, but hepatic function did not recover and liver failure progressed. Therefore, he underwent LDLT using an S2 monosegment graft at age 1.3 years. The patient had a leak at the Roux-en-Y anastomosis, which was repaired. Despite occasional episodes of pneumonia and otitis media, he is generally doing well 6 years after LDLT with continued immunosuppression agents. In conclusion, the combination of hematopoietic stem cell transplantation (HSCT) and liver transplantation may be efficacious, and HSCT should precede liver transplantation for children with X-linked SCID and liver failure.

Meta-analysis and meta-regression of outcomes for adult living donor liver transplantation versus deceased donor liver transplantation.

Prior single center or registry studies have shown that living donor liver transplantation (LDLT) decreases waitlist mortality and offers superior patient survival over deceased donor liver transplantation (DDLT). The aim of this study was to compare outcomes for adult LDLT and DDLT via systematic review. A meta-analysis was conducted to examine patient survival and graft survival, MELD, waiting time, technical complications, and postoperative infections. Out of 8600 abstracts, 19 international studies comparing adult LDLT and DDLT published between 1/2005 and 12/2017 were included. U.S. outcomes were analyzed using registry data. Overall, 4571 LDLT and 66,826 DDLT patients were examined. LDLT was associated with lower mortality at 1, 3, and 5 years posttransplant (5-year HR 0.87 [95% CI 0.81-0.93], p < .0001), similar graft survival, lower MELD at transplant (p < .04), shorter waiting time (p < .0001), and lower risk of rejection (p = .02), with a higher risk of biliary complications (OR 2.14, p < .0001). No differences were observed in rates of hepatic artery thrombosis. In meta-regression analysis, MELD difference was significantly associated with posttransplant survival (R2 0.56, p = .02). In conclusion, LDLT is associated with improved patient survival, less waiting time, and lower MELD at LT, despite posing a higher risk of biliary complications that did not affect survival posttransplant.

Validation of the Model for End-stage Liver Disease sodium (MELD-Na) score in the Eurotransplant region.

The MELD score is used in the Eurotransplant (ET) region to allocate liver grafts. Hyponatremia in cirrhotic patients is an important predictor of death but is not incorporated in MELD. This study investigated the performance of the MELD-Na score for the ET region. All adult patients with chronic liver disease on the ET liver transplantation waiting list (WL) allocated through lab MELD scores were included. The MELD-corrected effect of serum sodium (Na) concentration at listing on the 90-day WL mortality was calculated using Cox regression. The MELD-Na performance was assessed with c-indices, calibration per decile and Brier scores. The reclassification from MELD to MELD-Na score was calculated to estimate the impact of MELD-Na-based allocation in the ET region. For the 5223 included patients, the risk of 90-day WL death was 2.9 times higher for hyponatremic patients. The MELD-Na had a significantly higher c-index of 0.847 (SE 0.007) and more accurate 90-day mortality prediction compared to MELD (Brier score of 0.059 vs 0.061). It was estimated that using MELD-Na would reduce WL mortality by 4.9%. The MELD-Na score yielded improved prediction of 90-day WL mortality in the ET region and using MELD-Na for liver allocation will very likely reduce WL mortality.

The lectin-like domain of thrombomodulin is a drug candidate for both prophylaxis and treatment of liver ischemia and reperfusion injury in mice.

Ischemia and reperfusion injury (IRI) can occur in any tissue or organ. With respect to liver transplantation, the liver grafts from donors by definition experience transient ischemia and subsequent blood reflow. IRI is a problem not only in organ transplantation but also in cases of thrombosis or circulatory disorders such as mesenteric ischemia, myocardial, or cerebral infarction. We have reported that recombinant human soluble thrombomodulin (rTM), which is currently used in Japan to treat disseminated intravascular coagulation (DIC), has a protective effect and suppresses liver IRI in mice. However, rTM may not be fully safe to use in humans because of its inherent anticoagulant activity. In the present study, we used a mouse liver IRI model to explore the possibility that the isolated lectin-like domain of rTM (rTMD1), which has no anticoagulant activity, could be effective as a therapeutic modality for IRI. Our results indicated that rTMD1 could suppress ischemia and reperfusion-induced liver damage in a dose-dependent manner without concern of associated hemorrhage. Surprisingly, rTMD1 suppressed the liver damage even after IR insult had occurred. Taken together, we conclude that rTMD1 may be a candidate drug for prevention of and therapy for human liver IRI without the possible risk of hemorrhage.

Sleeve gastrectomy prior to liver transplantation is superior to medical weight loss in reducing posttransplant metabolic complications.

Strategies to optimize the management of obesity-related metabolic complications after liver transplantation (LT) are needed. We examined the effect of pre-LT sleeve gastrectomy (SG), as compared to medical weight loss (MWL), on post-LT outcomes. This is a cohort study of adults (≥18 years) with medically complicated obesity who were eligible for pre-LT SG and underwent LT from January 1, 2006 to June 1, 2016. Logistic regression models evaluated the association of SG on post-LT diabetes and hypertension, defined as new-onset or progressive disease post-LT. Cox regression models evaluated the association of SG on recurrent and de novo nonalcoholic fatty liver disease (NAFLD). Among 70 LT recipients who were eligible for pre-LT SG, 14 (20%) underwent SG and 56 (80%) underwent MWL only. Mean follow-up was 5.2 years post-LT. The SG cohort sustained higher % total body weight loss at 3 years post-LT (28.9% vs. 5.4%, p < .001). In multivariable analyses, SG was associated with significantly lower risk of post-LT diabetes (OR 0.04, 95% CI 0.00-0.41, p = .01), hypertension (OR 0.15, 95% CI 0.04-0.67, p = .01), and recurrent and de novo NAFLD (HR 0.19, 95% CI 0.04-0.91, p = .04). When compared to MWL, SG resulted in sustained weight loss and significantly lower risk of diabetes, hypertension, and recurrent and de novo NAFLD post-LT.

Changes and mediators of survival disparity among Black liver transplant recipients in the United States.

A landmark 2002 study identified Black liver transplant (LT) recipients as having lower post-LT survival compared to other races. While persistent disparities exist, changes over time and mediating factors are understudied. Capturing LT recipients between 2002 and 2018 in UNOS, we used logistic regression and Cox proportional-hazard models to calculate differences in post-LT mortality among races. We examined interactions between transplant year and race. A mediation analysis assessed biologic and environmental factors potentially associated with race differences in post-LT survival. The cohort included 46,997 LT recipients (3898 Black;36,560 White;6539 Hispanic). In most years, Black (vs. White) LT recipients had a higher probability of age-adjusted mortality, not observed among Hispanics. In multivariable analysis, Blacks (vs. Whites) had higher (aHR = 1.15, 95% CI 1.07-1.24), whereas Hispanics had lower (aHR = 0.78, 95% CI 0.72-0.83) risk of mortality. Differences in post-LT mortality among Blacks (vs. Whites) narrowed between 2002 and 2009, were similar between 2010 and 2013, and may have worsened between 2014 and 2018. Race differences were larger for mortality beyond 1-year post-LT (vs. within 1-year), and among non-HCV (vs. HCV). Alcohol-associated liver disease (ALD) was the strongest mediator (13.9%, 95% CI 8.7-32.7%) of the Black-White disparity in 2010-2018. Our analyses suggest disparities may worsen with longer follow-up, as HCV recedes with elimination efforts, and with further increases in ALD.

Center variation in long-term outcomes for socioeconomically deprived children.

Neighborhood socioeconomic deprivation is associated with adverse outcomes after pediatric liver transplant. We sought to determine if this relationship varies by transplant center. Using SRTR, we included patients <18 years transplanted 2008-2013 (N = 2804). We matched patient ZIP codes to a deprivation index (range [0,1]; higher values indicate increased socioeconomic deprivation). A center-level patient-mix deprivation index was defined by the distribution of patient-level deprivation. Centers (n = 66) were classified as high or low deprivation if their patient-mix deprivation index was above or below the median across centers. Center quality was classified as low or high graft failure if graft survival rates were better or worse than the overall 10-year graft survival rate. Primary outcome was patient-level graft survival. We used random-effect Cox models to evaluate center-level covariates on graft failure. We modeled center quality using stratified Cox models. In multivariate analysis, each 0.1 increase in the patient-mix deprivation index was associated with increased hazard of graft failure (HR 1.32; 95%CI: 1.05, 1.66). When stratified by center quality, patient-mix deprivation was no longer significant (HR 1.07, 95%CI: 0.89, 1.28). Some transplant centers care for predominantly high deprivation children and maintain excellent outcomes. Revealing and replicating these centers' practice patterns should enable more equitable outcomes.

Retrospective-prospective study of safety and efficacy of sofosbuvir-based direct-acting antivirals in HIV/HCV-coinfected participants with decompensated liver disease pre- or post-liver transplant.

Direct-acting antiviral (DAA) therapy has transformed the management of human immunodeficiency virus (HIV) and hepatitis C (HCV) coinfected patients with advanced liver disease. STOP-Coinfection was a multicenter prospective and retrospective, open-label study using sofosbuvir-based DAA therapy to treat HIV/HCV-coinfected participants pre- or post-liver transplant (LT). Sixty-eight participants with end-stage liver disease (Child-Turcotte-Pugh score ≥7 and Model for End-Stage Liver Disease score 6-29) were enrolled, 26 had hepatocellular carcinoma. Forty-two participants were treated pre-LT and 26 post-LT. All participants completed therapy without need for dose reduction or transfusion; eight required two or more courses of therapy. Ninety-three percent achieved a sustained virologic response and DAA therapy was well tolerated. Despite HCV cure, 12 end-stage liver disease participants required subsequent LT, 7 for decompensated liver disease. Thirteen participants died, 10 with decompensated liver disease pre-LT and three post-LT. Overall, transplant free survival was 42.8% at 4 years and post-LT survival was 87.9% at 5 years. We conclude that sofosbuvir-based DAA therapy is safe and highly effective in HCV-HIV patients with decompensated liver disease and post-LT, with post-LT survival rates comparable to other indications. This removes one of the last barriers to liver transplantation in this challenging cohort of recipients.

Liver transplantation for acute liver failure in a SARS-CoV-2 PCR-positive patient.

Current guidelines recommend deferring liver transplantation (LT) in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection until clinical improvement occurs and two PCR tests collected at least 24 hours apart are negative. We report a case of an 18-year-old, previously healthy African-American woman diagnosed with COVID-19, who presents with acute liver failure (ALF) requiring urgent LT in the context of SARS-CoV-2 polymerase chain reaction (PCR) positivity. The patient was thought to have acute Wilsonian crisis on the basis of hemolytic anemia, alkaline phosphatase:bilirubin ratio <4, AST:ALT ratio >2.2, elevated serum copper, and low uric acid, although an unusual presentation of COVID-19 causing ALF could not be excluded. After meeting criteria for status 1a listing, the patient underwent successful LT, despite ongoing SARS-CoV-2 PCR positivity. Remdesivir was given immediately posttransplant, and mycophenolate mofetil was withheld initially and the SARS-CoV-2 PCR test eventually became negative. Three months following transplantation, the patient has made a near-complete recovery. This case highlights that COVID-19 with SARS-CoV-2 PCR positivity may not be an absolute contraindication for transplantation in ALF. Criteria for patient selection and timing of LT amid the COVID-19 pandemic need to be validated in future studies.

Successfully sharing the sandbox: A perspective on combined DCD liver and heart donor procurement.

Donation after circulatory death (DCD) heart transplantation is currently being performed in the United States as part of a clinical trial. As with all donor procurements, effective coordination between all involved teams is vital for successful organ recovery and maximal utilization of donor organs. The current discussion relays a viewpoint on combined DCD liver and heart donor procurement. Key issues highlighted include the vital importance of donor warm ischemia time (DWIT) on outcome for both recipients as well as issues pertaining to DWIT that may arise when performing combined DCD liver and heart donor procurement.

Clinical parameters and biomarkers predicting spontaneous operational tolerance after liver transplantation: A scoping review.

Indefinite allograft acceptance after immunosuppression withdrawal (ISW), also known as operational tolerance (OT), can occur spontaneously after liver transplantation (LT), but reliable and reproducible prognosis of OT versus non-OT outcomes remains elusive. To prime this, systematic extraction of OT-predictive factors from the literature is crucial. We provide the first comprehensive identification and synthesis of clinical parameters and biomarkers predicting spontaneous OT in non-autoimmune/non-replicative viral LT recipients selected for ISW. We searched Embase, Medline, the Cochrane Central Register of Controlled Trials, clinicaltrials.gov, and the World Health Organization International Clinical Trials Registry Platform for articles, conference abstracts, and ongoing trials. We contacted principal investigators of stand-alone abstracts and ongoing trials for unpublished data and screened citations and references of eligible articles. Twenty-three articles reporting on 11 completed ISW studies, 13 abstracts, and five trial registry entries were included. Longer time between LT and ISW was the only clinical parameter that may increase the incidence of OT. Prognostic biomarkers conspicuously differed between pediatric and adult ISW candidates. These included allograft gene expression patterns and peripheral blood immune exhaustion markers for adults, and histological allograft scores for children. Our results will foster cross-validation efforts to facilitate safe and harmonized candidate selection for successful ISW.

Transplant trends in Mexico during the COVID-19 pandemic: Disparities within healthcare sectors.

Healthcare systems worldwide were challenged during the COVID-19 pandemic. In Mexico, the public hospitals that perform most transplants were adapted to provide care for COVID-19 patients. Using a nationwide database, we describe the first report of the impact of COVID-19 and related transplantation healthcare policies in a middle-income country by comparing statistics before and during the pandemic (pre-COVID: March 2019-February 2020 vs. COVID era: March 2020-February 2021) and by type of institution (public vs. private). The global reduction in transplantation was higher in public institutions compared with private institutions, 89% versus 62%, respectively, p < .001. When analyzing by organ, kidney transplantation decreased by 89% at public versus 57% at private, p < .001; cornea by 88% at public versus 64% at private, p < .001; liver by 88% at public versus 35% at private, p < .001; and heart by 88% in public versus 67% at private institutions, p = .4. The COVID-19 pandemic along with the implemented health policies were associated with a decrease in donations, waiting list additions, and a decrease in transplantation, particularly at public institutions, which care for the most vulnerable.

Outcomes of status 1 liver transplantation for Budd-Chiari Syndrome with fulminant hepatic failure.

There is a paucity of data on the outcome of liver transplantation (LT) in Budd-Chiari Syndrome (BCS) patients who are listed as status 1. The objective of our study was to determine patient or graft survival following LT in status 1 BCS patients. We utilized United Network for Organ Sharing (UNOS) database to identify all adult patients (> 18 years of age) listed as status 1 with a primary diagnosis of BCS in the United States from 1998 to 2018, and analyzed their outcomes and compared it to non-status 1 BCS patients. Four hundred and forty-six patients with BCS underwent LT between 1998 and 2018, and of these 55 (12.3%) were listed as status 1. There was no difference in long-term post-liver transplant or "intention-to-treat" survival from the time of listing to death or the last day of follow-up between status 1 and non-status 1 groups. Graft and patient survival at 5 years for status 1 patients were 75% and 82%, respectively. Cox regression analysis showed that patients listed as status 1 (aHR: 0.45, p < .02) were associated with a better survival. BCS patients listed as status 1 have excellent survival following emergency LT.

SARS-CoV-2-specific serological and functional T cell immune responses during acute and early COVID-19 convalescence in solid organ transplant patients.

The description of protective humoral and T cell immune responses specific against SARS-CoV-2 has been reported among immunocompetent (IC) individuals developing COVID-19 infection. However, its characterization and determinants of poorer outcomes among the at-risk solid organ transplant (SOT) patient population have not been thoroughly investigated. Cytokine-producing T cell responses, such as IFN-γ, IL-2, IFN-γ/IL-2, IL-6, IL-21, and IL-5, against main immunogenic SARS-CoV-2 antigens and IgM/IgG serological immunity were tracked in SOT (n = 28) during acute infection and at two consecutive time points over the following 40 days of convalescence and were compared to matched IC (n = 16) patients admitted with similar moderate/severe COVID-19. We describe the development of a robust serological and functional T cell immune responses against SARS-CoV-2 among SOT patients, similar to IC patients during early convalescence. However, at the infection onset, SOT displayed lower IgG seroconversion rates (77% vs. 100%; p = .044), despite no differences on IgG titers, and a trend toward decreased SARS-CoV-2-reactive T cell frequencies, especially against the membrane protein (7 [0-34] vs. 113 [15-245], p = .011, 2 [0-9] vs. 45 [5-74], p = .009, and 0 [0-2] vs. 13 [1-24], p = .020, IFN-γ, IL-2, and IFN-γ/IL-2 spots, respectively). In summary, our data suggest that despite a certain initial delay, SOT population achieve comparable functional immune responses than the general population after moderate/severe COVID-19.